Later on it breaks up into isolated attacks of fever, each lasting from a few hours to a day or two, and separated from the next attack by fever-free intervals. This kind of fever is called intermittent fever, and the intervals are called intermissions.

It often happens that the attacks of intermittent fever succeed each other with great regularity. When they come on every day, we have a quotidian fever; when they come on every other day, we have a tertian fever; and where they occur every third day, a quartan fever.

It was shown by Torti some centuries ago that a remittent fever is really nothing but an intermittent fever in which the successive attacks are so frequent or prolonged that, so to speak, they run together and produce a continuous fever. Most patients suffer at first from the remittent type of fever, which, after a week or more, tends to break up into the intermittent type.

We should here be careful to note that the symptoms may be much modified if the patient has been taking quinine. Large doses of this drug will generally cut short the succession of attacks; while moderate doses will shorten them and prolong the intermissions. Thus a patient who has been taking quinine as a precautionary measure may have only a few slight attacks of fever, in place of the severe illness described above.

If the patient cannot or does not take sufficient quinine, the progress of the case is often as follows:— After some weeks (if the patient lives) the fever goes away of itself, leaving the sufferer very weak and anaemic. It may be absent for weeks or months, during which he often flatters himself that he is cured; but suddenly it comes on again as violently as before. This is called a relapse; and it is followed by another period

of improvement called a rally. These rallies and relapses may follow each other for years, even long after the patient has returned to a healthy climate, such as that of England.

If the patient is on the road to recovery, the relapses tend to become more and more slight, sometimes resulting in only a single short attack of fever. At other times, they may be alarmingly severe and may produce unconsciousness, or blackwater fever, or death. This is especially apt to occur if the patient has not obeyed his doctor's orders with sufficient care.

Relapses seem often to be provoked by such things as exposure to the sun, or chill, or fatigue, or other illness, or even indigestion; but the fundamental cause of them is the persistence of the malaria germs in the blood.

If the attacks have been very severe or the relapses have been occurring for a long time, the patient often falls into the condition called malarial cachexia, which is characterized by an enlarged spleen, anaemia, indigestion, and a dusky complexion.

Blackwater fever is a peculiarly dangerous kind of relapse, found chiefly in intensely malarious localities such as parts of Africa and the Terai in India. The patient has generally high fever, with a great deal of vomiting. The urine becomes very dark in colour, sometimes almost black; and there is usually considerable jaundice after a day or two. This condition rarely occurs except in persons who have neglected continuous treatment, and who have already suffered numerous ordinary attacks of fever. It is certainly dangerous, but by no means necessarily fatal; and patients often have several successive attacks of blackwater fever, and yet live through them all.

III. THE GERM

Such are the symptoms of malarial fever; but what is the cause of the disease? This was discovered by Laveran in 1880. Malarial fever is due to the presence in the blood of millions of minute microbes.

These little organisms are not bacteria, but are animal parasites. Each inhabits one of the red corpuscles of the blood, in which it lives just as a grub lives in a nut. Their size is less than the 1-2500th of an inch in diameter; and I compute that something like a quarter of a billion of them must be present to produce fever. If they are sufficiently numerous, they can easily be detected by means of a powerful microscope in a small drop of blood obtained from a patient's finger. They are called Haemamoebae. Closely allied parasites are found in monkeys, bats, and birds.

Laveran, Golgi, and MacCallum discovered the leading facts about them. They propagate in the blood by forming spores. But there is this peculiarity about them, that all the Hacmamoebae in a patient tend to produce their spores at the same time; and it is precisely at the moment when the spores of all these millions of little creatures are scattered in the blood that the patient's fever begins. Afterwards, when the young spores occupy fresh blood corpuscles, the fever ceases for a time. But when the new generation is matured and forms spores in its turn, a fresh attack of fever occurs. This explains the periodical attacks which are so characteristic of malarial fever.

There are at least three kinds of Haemamoebae

found in man. One forms spores every three days and causes quartan fever. Another forms spores every other day and produces tertian fever. The third kind

is the usual cause of the severe, and sometimes irregular, fevers of the tropics. Quotidian, or daily fever, may be caused by all the varieties, in a manner which cannot be discussed here. Patients often have two kinds in their blood at the same time.

These facts have been fully substantiated by the most careful investigations carried out by scores of scientific men in numerous countries. The literature of the subject consists of many hundreds of books and monographs.

The number of parasites in a patient varies from time to time. The more numerous they are, the worse as a rule is his fever. During the relapses, they are generally numerous enough to be found in a very small drop of blood; but when there has been no fever for some days, their numbers tend to diminish. But even when they cannot be found in a single drop of blood, we must not assume they have entirely died out of the patient. On the contrary, we are certain that they sometimes continue to live in a patient in comparatively small numbers for years-just as rats will live in a ship; and it is only when their numbers again increase to a quarter of a billion or more that another relapse comes When complete recovery occurs the parasites die out entirely; and this sometimes happens spontaneously.

on.

Some patients seem to become so accustomed to the presence of the parasites that they cease to suffer fever or other illness in consequence of them. This is especially the case with native children. Koch, Daniels, and others found that more than half the negro children in parts of Africa contain the parasites and yet appear to be quite well. Nevertheless, though these children do not themselves suffer much, they are the principal scource of infection for others. When the

children reach puberty the parasites tend to die out in them completely by some natural but unknown process.

A skilful microscopist will not only find the parasites in blood taken from a patient's finger, but will be able to detect at sight the species to which it belongs, the stage of progress which it has reached, and even its sex-for, in a certain sense, even these minute animals undoubtedly possess sex. It is easy, then, to verify the frequently recorded observation that the parasites largely decrease in number after the patient has been given a considerable dose of quinine. Indeed, we can state it as a fact-though we cannot quite explain the fact that quinine kills the parasites.

IV. How PARASITES IN GENERAL ARE COMMUNICATED FROM ONE HOST TO ANOTHER

All plants and animals possess parasites, and thousands of different species of parasites have been closely studied by science; we therefore know much about their general ways of life. As a rule, a particular species of parasite can live only in the particular species of animal in which, by the evolution of ages, it has acquired the power of living. It is therefore not enough for the parasites of an individual animal—say a man— to be able to multiply within that individual, but they must also make arrangements, so to speak, for their progeny to enter into and infect other individuals of the same species. They cannot live for ever in one individual; they must spread in some way or other to other individuals.

The shifts made by parasites to meet this requirement of their nature are many and various, and consti