This is the classification by Scheube. It is an arrangement which follows no definite plan save that of general convenience. Thus, malaria and sleeping-sickness are caused by closely related animal blood-parasites, and from one point of view should be classed together. In this place only the more general features of tropical diseases can be discussed, many of the forms enumerated having been treated under their respective headings.
The Malaria of the Tropics is of special significance. It is known that there is more than one kind of malaria ; in fact, two or three different blood parasites causing this disease have been described. These parasites are conveyed to man by the bites of certain kinds of mosquitoes—namely, members of a definite genus (Anopheles), of which some eighty or more species are known. Only a small number of these, however, are known to be malaria carriers. The disease does not originate in the mosquito ; it must come from some infected human being. As Manson has well put it : " Let us imagine some primitive native community, a village in some district where anopheles mosquitoes abound, any small native village of Africa or the Philippines or in South or Central America. Let it be supposed that this village is free from malaria. A stranger with malarial parasites enters this village. The local anopheles bite him, and become infected and infective. In a very short time every individual in the village gets malaria. Most recover, but some die. The survivors gradually become immune after many years and many inoculations. Children are born, however, and so the disease goes on, the older inhabitants immune, but the disease kept going through the medium of the children. This is the actual condition of every' village in a malarious district." The great lesson to be learned is that a native village should be shunned, especially if it is in a district where the anopheles abound. This is particularly true at certain times of the day, notably at dusk, at night, and in the early morning hours, for at these times the mosquitoes feed. • The early morning hours are not unhealthy because they are early morning hours, but because the mosquito is abroad then. A railway crossing is dangerous only when there is a train passing. The adult native from a malarial district will make a safe guide and companion in the tropics, for he will not have malarial parasites ; but the youth, or the adult from a previously healthy neighbourhood, is not as safe a companion to choose, because of his liability to become infected. The essential thing to do in a malarial
district is to prevent the mosquitoes from becoming infected by biting malarial patients, and the disease cannot spread. Every malarial patient should be actively treated with quinine and kept in a mosquito-proof room.
The malarial parasite may live in the human body at least a year or, in some cases, two. After this it is apt to die out unless a re-infection takes place. In the tropics malaria exists in its most virulent form. It is called by a vast variety of names : Swamp fever, marsh fever, paludal fever, jungle fever, hill fever, coast fever, mountain fever, gnat fever ; and also by local names in abundance : Daar, Batavia, Kamerun, Panama fever, etc. It is the most common of all the tropical fevers, and most people who live in the tropics sooner or later acquire malarial fever. It is not, however, the only tropical fever by any means. Other chronic fevers may be due to the Mediterranean fever, liver abscess, elephantiasis, trypanosomiasis, kala azar, relapsing fever, or even leprosy. Three important features are con cerned in the diagnosis : Periodicity, microscopical appearance of parasites in the blood, and the reaction to quinine. Any fever recurring every forty eight or seventy-two hours is usually malarial, although mistakes in diagnosis are very frequent in the presence of seventy-two hour periodicity of the fever. It may be an abscess and a sign of pus instead of malaria. For the special symptoms of MALARIA consult the article under that heading.
Yellow Fever is an epidemic tropical disease of unknown causation, although it is not improbable that it is due to some form of minute animal, and communicated and spread also by means of a mosquito host. The life history of yellow fever has not yet been thoroughly worked out, but it seems fairly definitely proved that it is conveyed by a mosquito of the genus Stegomyia (S. fasciata). The old-fashioned methods of disinfection for yellow fever are, therefore, inadmissible ; and the chief concern of the hygienists of the future, in their attempts to diminish the spread of this disease, will be to kill the Stegomyia and also to limit the infection of this or other conveying animal hosts by carefully screening off the patients. The chief symptoms of yellow fever are considered under the alphabetical arrangement.