Insects and disease; a popular account of the way in which insects may spread or cause some of our common diseases, with many original illustrations from photographs

Preface

The subject of preventive medicine is one that is attracting world-wide attention to-day. We can hardly pick up a newspaper or magazine without seeing the subject discussed in some of its phases, and during the last few years several books have appeared devoted wholly or in part to the ways of preventing rather than curing many of our ills.

Looking over the titles of these articles and books the reader will at once be impressed with the importance that is being given to the subject of the relation of insects to some of our common diseases. As many of these maladies are caused by minute parasites or microbes the zoologists, biologists and physicians are studying with untiring zeal to learn what they can in regard to the development and habits of these organisms, and the entomologists are doing their part by studying in minute detail the structure and life-history of the insects that are concerned. Thus many important facts are being learned, many important observations made. The results of the best of these investigations are always published in technical magazines or papers that are usually accessible only to the specialist.

This little book is an attempt to bring together and iii iv Preface

place in untechnical form the most important of these facts gathered from sources many of which are at present inaccessible to the general reader, perhaps even to many physicians and entomologists.

In order that the reader who is not a specialist in medicine or entomology may more readily understand the intimate biological relations of the animals and parasites to be discussed it seems desirable to call attention first to their systematic relations and to review some of the important general facts in regard to their structure and life-history. This, it is believed, will make even the most complex special interrelations of some of these organisms readily understandable by all. Those who are already more or less familiar with these things may find the bibliography of use for more extended reading.

My thanks are due to Prof. V. L. Kellogg for reading the manuscript and offering helpful suggestions and criticisms.

Unless otherwise credited the pictures are from photographs taken by the author in the laboratory and field. As many of these are pictures of live specimens it is believed that they will be of interest as showing the insects, not as we think they should be, but as they actually are. Mr. J. H. Paine has given me valuable aid in preparing these photographs.

R. W. D.

Stanford University, California,

Chapter I Page

Parasitism and Disease i

Definition of a parasite, i; examples among various animals, 2; Parasitism, 3; effect on the parasite, 4; how a harmless kind may become harmful, 5; immunity, 6; Diseases caused by parasites, 7; ancient and modern views, 7; Infectious and contagious diseases, 8; examples, 9; importance of distinguishing, 9; Effect of the parasite on the host, 9; microbes everywhere, 10; importance of size, 11; numbers, 11; location, 11; mechanical injury, 12; morphological injury, 13; physiological effect, 13; the point of view, 14.

Chapter II

Bacteria, 15; border line between plants and animals, 15; most bacteria not harmful, 15; a few cause disease, 15; how they multiply, 15; parasitic and nonparasitic kinds, 17; how a kind normally harmless may become harmful, 18; effect of the bacteria on the host, 18; methods of dissemination, 18. Protozoa, 19; Amoeba, 19; its lack of special organs, 19; where it lives, 19; growth and reproduction, 19; Classes of Protozoa, 20; the amoeba-like forms, 20; the flagellate forms, 20; importance of these, 21; the ciliated forms, 22; the Sporozoa or spore-forming kinds, 22; these most important, 23; abundance, 23; adaptability, 23; common characters, 24; ability to resist unfavorable conditions, 24.

v vi Contents

Chapter III Page

Ticks, 26; general characters, 27; mouth-parts, 27; habits, 27; life-history, 27; Ticks and disease, 28; Texas fever, 28; its occurrence in the north, 28; carried by a tick, 29; loss and methods of control, 31; other diseases of cattle carried by ticks, 31; Rocky Mountain spotted fever, 32; its occurrence, 32; probably caused by parasites, 32; relation of ticks to this disease, 33; Relapsing Fever, 33; its occurrence, 34; transmitted by ticks, 34; Mites, 35; Face-mites, 35; Itch-mites, 36; Harvest-mites, 37.

Chapter IV

Numbers, 40; importance, 41; losses caused by insects, 41; loss of life, 42; The flies, 43; horse-flies, 43; stable-flies, 44; surra, 45; nagana, 45; black-flies, 46; punkies, 46; screw-worm flies, 47; blow-flies, 48; flesh-flies, 48; fly larvae in intestinal canal, 49; bot-flies, 50; Fleets, 52; jigger-flea, 53; Bedbugs, 54; Lice, 54; How insects may carry disease, 55; in a mechanical way, 55; as one of the necessary hosts of the parasite, 56.

Chapter V

The old attitude toward the house-fly, 57 ; its present standing, 58; reasons for the change, 58; Structure, 59; head and mouth-parts, 60; thorax and wings, 61; feet, 62; How they carry bacteria, 62; Life-history, 63; eggs, 63; ordinarily laid in manure, 63; other places, 63; habits of the larvae, 64; habits of the adults, 64; places they visit, 65; Flies and typhoid, 65; patients carrying the germs before and after they have had the disease, 65; how the flies get these on their body and distribute them, 66; results of some observations and experiments, 66; Flies and other diseases, 68; flies and cholera, 68; flies and tu-

Contents vii berculosis, 69; possibility of their carrying other diseases, 70; Fighting flies, 71; screens not sufficient, 71; the larger problem, 71; the manure pile, 72; outdoor privies, 72; garbage can, 72; cooperation necessary, 72; city ordinances, 73; an expert's opinion of the house-fly, 73; Other flies, 75; habits of several much the same but do not enter house as much, 75; the small house-fly, 75; stable-flies, 75; these may spread disease, 75.

Chapter VI

Numbers, 76; interest and importance, 76; eggs, 77; always in water, 77; time of hatching, 77; Larva, 78; live only in water, 78; head and mouth -parts of larvae, 78; what they feed on, 78; breathing apparatus, 79; growth of the larvae, 80; Vupce, 80; active but takes no food, 80; breathing tubes, 80; how the adult issues, 81; The Adult, 81; male and female, 81; how mosquitoes "sing" and how the song is heard, 82; the palpi, 82; The Mouth-parts, 2>y, needles for piercing, 8^; How the mosquito bites, 84; secretion from the salivary gland, 84; why males cannot bite, 84; blood not necessary for either sex, 84; The Thorax, 85; the legs, 85; the wings, 85; the balancers, 85; the breathing pores, 86; The abdomen, 86; The digestive system, 86; The salivary glands, 87; their importance, 87; effects of a mosquito bite, 87; probable function of the saliva, 88; How mosquitoes breathe, 89; Blood, 90; in body cavity, 90; heart, 90; Classification, 91; Anopheles, 91; distinguishing characters, 92; eggs, 92; where the larvae are found, 93; Yellow fever mosquito, 94; its importance, 94; the adult, 95; habits, 95; habits of the larvae, 95; Other species, 96; some in fresh water, others in brackish water, 96; Natural enemies of mosquitoes, 97; how natural enemies of mosquitoes control their numbers, 98; mosquitoes in Hawaii, 98; Enemies of the adults, 99; Enemies of the larvcB and pupae, 100; Fighting mosquitoes, 101; fighting the adult, 102; Fighting the larva, 103; domestic or local species, 104; draining and treating with oil, 104; combatting salt-marsh species by draining, 105; by minnows or oil, 105.

viii Contents

Chapter VII Page

Early reference to malaria, 106; its general distribution, 106; theories in regard to its cause, 107; insects early suspected, 107; The parasite that causes malaria, 108; studies of the parasite, 108; Life-history in human host, 109; its effect on the host, no; the search for the sexual generation, in; The parasite in the mosquito, 112; review of whole life-history, 115; malaria transmitted only by mosquitoes, 115; Summary, 117; experimental proof, 118.

Chapter VIII

A disease of tropical or semi-tropical countries, 120; outbreaks in the United States, 120; parasite that causes the disease not known, 121; formerly regarded as a contagious disease, 122; The yellow fever commission, 123; Dr. Finlay's claim, 124; experiments made by the commission, 125; summary of results, 129; what it means, 130; results in Havana, 131; the fight in New Orleans, 132; In the Panama canal zone, 135; in Rio de Janiero, 136; claims of the French commission, 138; habits of stegomyia, 139; breeding habits, 139; possible results of war against the mosquitoes, 139; Danger of this disease in the Pacific Islands, 140.

Chapter IX

Great scourges, 142; the "black death," 142; old conditions and new, 143; How plague was controlled in San Francisco, 143; Indian Plague commission, 144; Dr. Simond's claim, 145; The advisory committee and the new commission, 146; Results of Dr. Virjbitski's experiments, 147 ; Results of various investigations, 150; structure and habits of fleas, 151; feeding habits, 152; Common species of fleas, 153; Ground squirrels and plague, 155; squirrel fleas, 156; Remedies for fleas, 157; cats and dogs, 159.

Contents ix

Chapter X Page

Other Diseases, Mostly Tropical, Known or Thought to

Be Transmitted by Insects 161

Sleeping Sickness, 161; its occurrence in Africa, 161; caused by a Protozoan parasite, 162; the tsetse-fly, 163; Elephantiasis, 164; caused by parasitic worms, 169; their development, 165; how they are transferred to man, 165; effect on the patient, 166; Dengue, 168; other names, 168; probably transmitted by mosquitoes, 170; Mediterranean fever, 171; cause, 171; may be conveyed by mosquitoes, 171; Leprosy, 171; caused by a bacteria parasite, 171; possibilities of flies, mosquitoes and other insects transmitting the disease, 172; Kala-azar, 173; transmitted by the bedbug, 173; Oriental sore, 174; the parasite may be carried by insects, 174.

Parasites and parasitism, 175; Protozoa, 176; Bacteria, 177; Insects and disease, 178; Mosquitoes — systematic and general, 179; Mosquito anatomy, 182; Mosquitoes — life-history and habits, 183; Mosquito fighting, 183; Mosquitoes and disease, 185; Malaria, 186; Yellow fever, 189; Dengue, 192; Filarial diseases and elephantiasis, 193; Leprosy, 193; Plague, 194; Fleas, 198; Typhoid fever, 199; House-flies — anatomy, life-history, habits, 200; House-flies and typhoid, 202; House-fly and various diseases, 203; Human myiasis, 207; Stomoxys and other flies, 208; tsetse-flies, 209; Typanosomes and Trypanosomiasis, 210; Sleeping sickness, 211; Rocky mountain fever and ticks, 212; Ticks and various diseases, 213; Kala-azar and bedbugs, 216; Text or reference books, 216; Miscellaneous articles, 218.

ILLUSTRATIONS

An artificial lake, nearly dry and partly ftlled with rubbish, has become a breeding- GROUND FOR DANGEROUS MOSQUITOES . . Frontispiece

PAGE

Fig. i. A lamprey 2 -

Fig. 2. Sacculina 2

Fig. 3. Trichina spiralis 2

Fig. 4. An external parasite, a bird-louse (Lipeurus

ferox) 3

Fig. 5. An internal parasite, a tachina fly {Blephari-

peza adusta) 3

Fig. 6. Work of an internal parasite, puss-moth larva

parasitized by a small ichneumon fly ... 3

Fig. 7. Typhoid fever bacilli 20 *

Fig. 8. Amceba 20

Fig. 9. Euglina virdis 21

Fig. 10. Spirocheta duttoni 21

Fig. 11. Paramecium 22.

Fig. 12. Vorticella 22

Fig. 13. Pathogenic protozoa; a group of intestinal

parasites 22

Fig. 14. Castor-bean tick {Ixodes ricinus) 28

Fig. 15. Texas fever tick 28

Fig. 16. Texas fever tick {Mar gar opus annulatus) . . T 29

Fig. 17. Amblyomma variegatum 29

Fig. 18. Ornithodoros moubata 36

Fig. 19. The follicle mite {Demodex folliculorum) ... 36

Fig. 20. Itch-mite {Sar copies scabiei) 37

Fig. 21. Harvest-mites or "jiggers" 37

Fig. 22. Horse-fly {Tabanus punctifer) 44 '

xi xii Illustrations

PAGE

Fig. 23. Stable-fly (Stomoxys calcitrans) 44

Fig. 24. A black-fly {Simulium sp.) 45

Fig. 25. Screw- worm fly {Chrysomyia macellaria) ... 45

Fig. 26. Blow-fly (Calliphora vomitoria) 45

Fig. 27. Bluebottle fly {Lucilia sericata) 50

Fig. 28. Flesh-fly (Sarcophaga sp.) 50

Fig. 29. "The little house-fly" {Homalomyia canicu-

laris) 51

Fig. 30. Horse bot-fly (Gastrophilus equi.) 51

Fig. 31. Ox- warble fly (Hypoderma lineata) 51

Fig. 32. Sheep bot-fly {Gastrophilus nasalis) 51

Fig. S3- Chigo or jigger-flea, male (Dermatophilus penetrans) 54

Fig. 34. Chigo, female distended with eggs 54

Fig. 35. Bedbug {Cimex lectularis) 55

Fig. 36. Body-louse {Pediculus vestimenti) 55

Fig. 37. One use for the house-fly 57

Fig. 38. The house-fly {Musca domestica) 58

Fig. 39. Head of house-fly showing eyes, antenna and

mouth-parts 60

Fig. 40. Proboscis of house-fly, side view 60

Fig. 41. Lobes at end of proboscis of house-fly showing

corrugated ridges 6l

Fig. 42. Wing of house-fly 61

Fig. 43. Wing of stable -fly {Stomoxys calcitrans) .... 62 Fig. 44. Wing of house-fly showing particles of dirt

adhering to it 62

Fig. 45. Last three segments of leg of house-fly ... 62

Fig. 46. Foot of house-flv 63

Fig. 47. Larva of house-fly 63

Fig. 48. Barnyard filled with manure 64

Fig. 49. Dirty stalls 65

Fig. 50. Pupa of house-fly 76

Fig. 51. Head of stable-fly 76

Fig. 52. Mass of mosquito eggs {Theobaldia incidens) . . 76

Fig. 53. Mosquito eggs and larvae {T. incidens) .... 77

Fig. 54. Mosquito larva {T. incidens), dorsal view ... 77

Illustrations xiii

PAGE

Fig. 55. Eggs, larv^: and pup,e of mosquitoes (T. inci-

dens) 78

Fig. 56. Larva of mosquito (T. incidens) 78

Fig. 57. Mosquito larvae and pup,e {T. incidens) .... 79

Fig. 58. Anopheles larvae (A. maculipennis) 79

Fig. 59. Mosquito pup.e (J. incidens) 80

Fig. 60. Mosquito pupa (T. incidens) 80

Fig. 61. Mosquito larvae and pup,e {T. incidens) .... 80

Fig. 62. A female mosquito (T. incidens) 81

Fig. 63. A male mosquito {T. incidens) 81

Fig. 64. Head and thorax of female mosquito (Ochlero-

tatus lativittatus) 82

Fig. 65. Head and thorax of male mosquito (0. lativittatus) 82

Fig. 66. Head of female mosquito 83

Fig. 67. Cross-section of proboscis of female and male

mosquito 83

Fig. 68. Wing of mosquito (0. lativittatus) 86

Fig. 69. End of mosquito wing highly magnified ... 86 Fig. 70. Diagram to show the alimentary canal and

salivary glands of a mosquito 87

Fig. 71. Salivary glands of mosquitoes 87

Fig. 72. Heads of culicln,e mosquitoes 90

Fig. 73. Heads of anophelin^e mosquitoes 90

Fig. 74. Wing of Anopheles maculipennis 90

Fig. 75. Wing of Theobaldia incidens 90

Fig. 76. A non-malarial mosquito (T. incidens), male,

standing on the wall 91

Fig. 77. Female of same 91

Fig. 78. A malarial mosquito (A. maculipennis), male,

standing on the wall 91

Fig. 79. Female of same 91

Fig. 80. Egg of Anopheles, side view 92

Fig. 81. Egg of anopheles, dorsal view 92

Fig. 82. Anopheles larvae 92

Fig. 83. Anopheles larv^: 93

Fig. 84. Anopheles larva, dorsal view 93 xiv Illustrations

PAGE

Fig. 85. Anopheles pup,e resting at surface of water . 93 Fig. 86. Salt-marsh mosquito (Ochlerotatus lativittatus);

MALE 98

Fig. 87. Salt-marsh mosquito (O. lativittatus); female . . 98

Fig. 89. Dragon-flies 99

Fig. 90. The young (nymph) of a dragon-fly 100

Fig. 91. The cast skin {exuvce) of a dragon-fly nymph . 100

Fig. 92. Diving-beetles and back-swimmers 101

Fig. 95. An old watering-trough, an excellent breeding-place FOR MOSQUITOES 103

Fig. 96. Horse and cattle tracks in mud filled with

WATER 108

Fig. 97. A malarial mosquito {Anopheles maculipennis)

MALE I08

Fig. 98. A malarial mosquito (A. maculipennis) female . 109 Fig. 99. Diagram to illustrate the life-history of the

malarial parasite iio

Fig. 100. Malarial mosquito (A. maculipennis) on the wall hi Fig. ioi. Malarial mosquito (A. maculipennis) standing on

A TABLE Ill

Fig. 102. Salt-marsh mosquito (O. lativittatus) standing on

A TABLE Il8

Fig. 103. Anopheles hanging from the ceiling .... 118

Fig. 104. Yellow fever mosquito (Stegomyia calopus) . . 122

Fig. 105. Rat-flea (Loemopsylla cheopis) ; male 152

Fig. 106. Rat-flea (L. cheopis) ; female 152

Fig. 107. Head of rat-flea showing mouth-parts . . . 153

Fig. 108. Human-flea (Pulex irritans) ; male 153

Fig. 109. Human-flea (P. irritans) ; female 156

Fig. ho. Mouse-flea (Ctenopsyllus musculi); female . . . 156

Fig. hi. Trypanosoma gambiense 164

Fig. 112. Tsetse-fly 164

Chapter I Parasitism And Disease Parasites

HE dictionary says that a parasite is a living organism, either animal or plant, that lives in or on some other organism from which it derives its nourishment for a whole or part of its existence. This definition will serve as well as any, as it seems to include all the forms that might be classed as parasites. As a general thing, however, we are accustomed to think of a parasite as working more or less injury to its host, or perhaps we had better say that if it does not cause any irritation or ill effects its presence is not noted and we do not think of it at all.

As a matter of fact the number of parasitic organisms that are actually detrimental to the welfare of their hosts is comparatively small while the number of forms both large and small that lead parasitic lives in or on various hosts, usually doing no appreciable harm, often perhaps without the host being aware of their presence, is very great indeed.

Few of the higher animals live parasitic lives. The nearest approach to a true parasite among the vertebrates is the lamprey-eel (Fig. i) which attaches itself to the body of a fish and sucks the blood or eats the flesh. Among the Crustaceans, the group that includes the lobsters and crabs, we find many examples of parasites, the most extraordinary of which is the curious crab known as Sacculina (Fig. 2). In its early stages this creature is free-swimming and looks not unlike other young crabs. But it soon attaches itself to another crab and begins to live at the expense of its host. Then it commences to undergo remarkable changes and finally becomes a mere sac-like organ with a number of long slender root-like processes penetrating and taking nourishment from the body of the unfortunate crab-host.

The worms furnish many well-known examples of parasites, whole groups of them being especially adapted to parasitic life. The tapeworms, common in many animals and often occurring in man, the roundworms of which the trichina (Fig. 3) that causes " measly" pork is a representative, are

Fig. i

Fig. 2

Fig. 3

Fig. i — A lamprey. (After Goode.)

Fig. 2 — Sacculina; A, parasite attached to a crab; B, the active larval condition; C, the adult removed from its host. (After Haeckel.)

Fig. 3 — Trichina spiralis encysted in muscle of a pig. (From Kellogg's Elementary Zool.)

V

Fig. 5

Fig. 4

Fig. 6

Fig. 4 — An external parasite, a bird-louse (Lipeurus ferox).

Y\G. 5 — A tachina fly {Blepharipeza adusta) the larva of which is an internal parasite.

FlG. 6— Work of an internal parasite, puss-moth larva parasitized by a small ichneumon fly.

familiar examples. These and a host of others al" show a very high degree of specialization fitting them for their peculiar lives in their hosts.

From among the insects may be selected interesting examples of almost all kinds and degrees of parasitism, temporary, permanent, external, internal (Figs. 4, 5, 6). Among them is found, too, that curious condition known as hyperparasitism where one animal, itself a parasite, is preyed upon by a still smaller parasite.

"The larger fleas have smaller fleas Upon their backs to bite um, These little fleas still smaller fleas And so ad infinitum."

Coming now to the minute, microscopic, one-celled animals, the Protozoa, we find entire groups of them that are living parasitic lives, depending wholly on one or more hosts for their existence. Many of these have a very remarkable life-history, living part of the time in one host, part in another. The malarial parasite and others that cause some of the diseases of man and domestic animals are among the most important of these.

PARASITISM

Among all these parasites, from the highest to the lowest the process that has fitted them for a parasitic life has been one of degeneration. While they may be specialized to an extreme degree in one direction they are usually found to have some of the parts or organs, which in closely related forms are well developed, atrophied or entirely wanting. As a rule this is a distinct advantage rather than a disadvantage to the parasite, for those parts or organs that are lost would be useless or even in the way in its special mode of life.

Then, too, the parasite often gives up all its independence and becomes wholly dependent on its host or hosts not only for its food but for its dissemination from one animal to another, in order that the species may not perish with the host. But in return for all this it has gained a life of ease, free from most of the dangers that beset the more independent animals, and is thus able to devote its whole time and energy to development and the propagation of the species.

We are accustomed to group the parasites that we know into two classes, as harmful or injurious and as harmless, the latter including all those kinds that do not ordinarily affect our well-being in any way. But such a classification is not always satisfactory or safe, for certain organisms that to-day or under present conditions are not harmful may, on account of a great increase in numbers or change of conditions, become of prime importance to-morrow. An animal that is well and strong may harbor large numbers of parasites which are living at the expense of some of the host's food or energy or comfort, yet the loss is so small that it is not noticed and the intruders, if they are thought of at all, are classed as harmless. Or we may at times even look upon them as beneficial in one way or another. "A reasonable amount of fleas is good for a dog. They keep him from brooding on being a dog."

But should these parasites for some reason or other increase rapidly they might work great harm to the host. Even David Harum would limit the number of fleas on a dog. Or the animal might become weakened from some cause so that the drain on its resources made by the parasites, even though they did not increase in numbers, would materially affect it.

Perhaps the most serious way in which parasites that are usually harmless may become of great importance is illustrated by their introduction into new regions or, as is more often the case, by the introduction of new hosts into the region where the parasites are found. Under normal conditions the animals of a given region are usually immune to the parasites of the same region. That is, they actually repel them and do not suffer them to exist in or on their bodies, or they are tolerant toward them. In the latter case the parasites live at the expense of the host, but the host has become used to their being there, adapted to them, and the injury that they do, if any, is negligible.

But when a new animal comes into the region from some other locality the parasites may be extremely dangerous to it. There are many striking examples of this. Most of the people living in what is known as the yellow fever belt are immune to the fever. They will not develop it even under conditions that would surely mean infection for a person from outside this zone. Certain of our common diseases which we regard as of little consequence become very serious matters when introduced among a people that has never known them before. The cattle of the southern states are immune to the Texas fever, but let northern cattle be sent south or let the ticks which transmit the disease be taken north where they can get on cattle there, and the results are disastrous.

Another striking example and one that is attracting world-wide attention just now is the trypanosome that is causing such devastation among the inhabitants of central Africa. With the advent of white men into this region and the consequent migration of the natives along the trade routes this parasite, which is the cause of sleeping sickness, is being introduced into new regions and thousands upon thousands of people are dying as a result of its ravages.

DISEASES CAUSED BY PARASITES

Some two hundred years ago, after it became known that minute animal parasites were associated with certain diseases and were the cause of them, it rapidly came to be believed that all our ills were in some way caused by such parasites, known or unknown. Further study and investigation failed to reveal the intruders in many instances and so it began to be doubted whether after all they were responsible for much that had been laid at their doors. Then after it was discovered that minute plant parasites, bacteria, were responsible for many diseases they in turn began to be accused of being the cause of most of the ills that the flesh is heir to.

In later years we have come to adopt what seems to be a more reasonable view, for we can see and definitely prove that neither of these extreme views was correct but that there was much truth in each of them. To-day we recognize that certain diseases, such as typhoid, cholera, tuberculosis and many others, are caused by the presence of bacteria in the body, and it is just as definitely known that such maladies as malaria and sleeping sickness are caused by animal parasites.

Then there is a long list of other epidemic diseases, such as smallpox, measles and scarlet fever, the exact cause of which has not been determined. Many of these are believed to be due to micro-organisms of some kind, and if so they will almost certainly sooner or later be found. Curiously enough most of the diseases in this last class and many of those in the first are contagious, while all that are caused by animal parasites are, as far as is known, infectious but not contagious.

INFECTIOUS AND CONTAGIOUS DISEASES

It is important that we keep in mind this distinction. By contagious diseases are meant those that are transmitted by contact with the diseased person either directly, by touch, or indirectly by the use of the same articles, by the breath or effluvial emanations from the body or other sources. Smallpox, measles, influenza, etc., are examples of this group. By infectious diseases are meant those which are disseminated indirectly, that is, in a roundabout way by means of water or food or other substances taken into or introduced into the body in some way. Typhoid, malaria, and yellow fever, cholera and others are examples of this class. Thus it is evident that all of the contagious diseases may be infectious, but many of the infectious diseases are not as a rule contagious, although some of them may become so under favorable conditions.

Just one example will show the importance of knowing whether a disease is contagious or infectious. Until a few years ago it was believed that yellow fever was highly contagious and every precaution was taken to keep the disease from spreading by keeping the infected region in strict quarantine. This often meant much hardship and suffering and always a great financial loss. We now know that it is infectious only and not contagious, and that all this quarantine was unnecessary. The whole fight in controlling an outbreak of yellow fever or in preventing such an outbreak is now directed against the mosquito, the sole agent by which the disease can be transmitted from one person to another.

EFFECT OF THE PARASITE ON THE HOST

We have seen how a few parasites in or on an animal do not as a rule produce any appreciable io Insects and Disease

ill effects. This is of course a most fortunate thing for us, for the parasitic germs are everywhere. There is perhaps "more truth than poetry" in the following newspaper jingle:

"Sing a song of microbes,

Dainty little things, Eyes and ears and horns and tails,

Claws and fangs and stings.

Microbes in the carpet,

Microbes in the wall, Microbes in the vestibule,

Microbes in the hall.

Microbes on my money,

Microbes in my hair, Microbes on my meat and bread,

Microbes everywhere.

Microbes in the butter,

Microbes in the cheese, Microbes on the knives and forks,

Microbes in the breeze.

Friends are little microbes,

Enemies are big, Life among the microbes is —

Nothing 'infra dig.' Fussy little microbes,

Millions at a birth, Make our flesh and blood and bones,

Keep us on the earth."

While of course most of these microbes are to be regarded as absolutely harmless and some as very useful, many have the power to do much injury if the proper conditions for their rapid development should at any time exist. While the size of the parasite is always a factor in the damage that it may do to the host the factor of numbers is perhaps of still greater importance because of the power of very rapid multiplication possessed by so many of the smaller forms.

Certain minute parasites in the blood may cause little or no inconvenience, but should they begin to multiply too rapidly some of the capillaries may be filled up and trouble thus result. Or take some of the larger forms. A few intestinal worms may cause no appreciable effect on the host, but as soon as their numbers increase serious conditions may come about simply by the presence of the great masses in the host even if they were not robbing it of its nourishment. Many instances are known where such worms have formed masses that completely clogged up the alimentary canal. Such injuries as these may be regarded as mechanical injuries. Some parasites injure the host only when they are laying their eggs or reproducing the young. These may clog up passages or some of them may be carried to some more sensitive part of the body where the damage is done. The guinea- worm of southwestern Asia and of Africa lives in the body of its host for nearly a year sometimes attaining a great length and migrating through the connective tissue to different parts of the body causing no particular inconvenience until it is ready to lay its eggs when it comes to the surface and then great suffering may result. The African eye-worm is another example of a parasite causing mechanical injury only at certain times. It works in the tissues of the body sometimes for a long while, doing no harm unless it finds its way to the connective tissue of the eyeball.