Man's Mortal Enemies 25 



disease, and if at any time we relax our vigilance, Yellow 

 Jack might slink out of the jungles and renew his deadly 

 march. 



Animals, Insects, and Man 



Everyone with a dog, a cat, a flock of chickens, or a herd 

 of cows knows that animals, as well as humans, get sick. But 

 we are inclined to overlook the fact that animals may suffer 

 from the same diseases that affect us. Since the infectious 

 nature of disease was first established, we have often fallen 

 into the trap of believing that we could always prevent the 

 spread of a disease by keeping the sick out of contact with the 

 well. But what good does that do if the disease is not 

 spread by personal contact but by means of an insect? And 

 worse yet, what if the insects don't carry the disease directly 

 from person to person but, instead, transfer it from some ani- 

 mal to a human? Maybe your pet dog is carrying a disease 

 that will some day be transferred to you. Don't say that's 

 impossible because your dog is in the best of health. Re- 

 member Typhoid Mary and all the other typhoid carriers 

 who show no symptoms of the disease yet can pass it on in 

 deadly form to others. 



Our knowledge of plague, typhus, malaria, and yellow 

 fever should be enough to convince us beyond doubt that our 

 animal population constitutes a reservoir of disease of appall- 

 ing proportions. But that is only part of the story. How 

 many other diseases come to us over the animal-insect-man 

 bridge? That's hard to say! For some diseases, the chain 

 of events has been proved; for others it is suspected; and for 

 still others, although there is as yet no evidence of an animal 

 origin, the possibility is by no means remote. How else can 



