Many Diseases Become Milder with Time

 

    Consider the spread of a frighteningly virulent disease like Ebola virus. Furthermore, consider it from the viewpoint of the virus. After infection the victim will most likely die in just a few days. Before the first victim dies, the virus must find another victim to infect. Obviously the longer the first victim stays alive and can move around, the greater the chances of the virus being brought into contact with someone else. If the virus kills or incapacitates the first victim too quickly, it will undermine its own chances of transmission.

 

    We should also consider the spread of the virus from village to village. As long as the virus stays in the same village, where there are plenty of potential victims close together, it can get away with killing fast. But what happens when the village has been wiped out? How can the virus get to the next center of human population? Only by being carried by an infected person who is still fit enough to travel. Over the long term, movement between population centers may be more critical than how a disease spreads locally within a group of people.

 

    Now consider two slightly different Ebola viruses. One kills in a day or two. The second allows its victims to survive a whole week. Clearly, virus number one may wipe out everyone within a crowded community, but it will find it very difficult to transfer itself to the next village. Even if a dying victim staggers within sight of the next village, its people will probably not allow him in. During plague epidemics in medieval Europe, many villages and small towns stationed archers to intercept travelers. Anyone showing symptoms of plague was warned away and shot if they ventured too close. While lacking in sensitivity, such quarantine measures were often effective and many small villages did escape entirely from epidemics that decimated nearby towns.

 

    The less virulent virus will spread much more effectively. Infected refugees fleeing an infected village may well have time to reach another center of population before symptoms appear. Thus, if we have a mixture of viruses, the milder forms will spread more effectively, and over time they will come to predominate. Many diseases appear to have done just this and have evolved to become much less aggressive. Examples include gonorrhea and syphilis (caused by bacteria), and measles, mumps and influenza (all caused by viruses). What unites these diseases is that they are all transmitted directly from person to person.

 

    Ebola virus infects the human population now and then after emerging from some animal host, probably bats. It wipes out a few people who are all in close contact and then the mini-epidemic burns itself out. Much the same is true of Lassa fever and other extremely virulent diseases that burst out of the jungle every so often. Although they provide the press with the opportunity for apocalyptic hand-wringing they are most unlikely to spread far without getting milder.

 

    If Ebola were to spread effectively in humans, it would have to mutate to a milder form. Such milder variants do in fact occur. "The Hot Zone", a book published a few years back, describes an outburst of an Ebola-type virus close to Washington DC. In 1989, a highly contagious disease attacked several hundred monkeys living in very crowded conditions in a facility in the suburb of Reston, Virginia. Over half the monkeys died in just a few days. However, only four people were infected. None of them died, or were even seriously ill. Although "Restonvirus" is a close relative of Ebola virus and still highly virulent for monkeys, it has mutated to become so mild in man that a couple of the human cases were almost symptomless. Perhaps the term "lukewarm zone" might have been more appropriate.

 

    Naturally acquired immunity is when you catch a disease and your body makes an immune response. The immune system will remember and if you are infected with the same disease agent later, you will be protected – i.e. you have become immune.

 

    The other way to get naturally acquired immunity is a special case. Antibodies can cross the placenta from mother to fetus and are also transferred from mother to baby through colostrum and breast milk. Colostrum is the first secretion of the mammary glands after birth. Milk is made later. Colostrum in particular has a lot of antibodies from the mother. Notice that in the mother to fetus or mother to baby transfer, the infant receives ready-made antibodies from somebody else.

 

    Despite the hype, viruses such as Hantavirus, Lassa fever and Ebola have had little global impact. About 50 million people die each year on planet Earth and of these about 16 million succumb to infectious disease. Malaria and TB account for roughly 3 million each and even measles still kills a million children a year in the Third World. Deaths per year from the newly emerging viruses number a mere few hundred. For example, between its emergence in 1976 and the present outbreak there have been approximately 1,000 official cases of Ebola virus infection with an overall death rate of 80%. Undoubtedly several thousand more victims have died unreported in isolated villages, nevertheless, in global terms these numbers are negligible.

 

    So should we all be alarmed about these novel viruses? Worried, yes, panicked, no. It's possible that someone infected with a new and virulent virus might board an aircraft in Africa and stopover for a day or two in London and New York on their way to Los Angeles. Despite being highly lethal, Ebola virus is not terribly infective, until the final stages when the blood is full of virus and the patient bleeds from all the bodily orifices. Obviously, at this stage the patient is immobile. Lassa and Hantavirus are much the same. Exposure to blood or tissue samples has resulted in infection of health and research workers, but casual contact rarely transfers the virus. During the 1995 outbreak in Zaire, of 28 relatives who stayed in hospital to help nurse the sick - often sharing the same room or even beds!! - 17 got Ebola. Of 78 who just visited, none fell ill. Similarly, in 1990 an American returning to Chicago from Nigeria was hospitalized and died of Lassa fever. No one else was infected, although no special precautions were taken and the infection was only identified as Lassa after he had died.

 

    Actually, none of Lassa, Ebola or Hantavirus are as evil as originally believed. Later investigations have shown that mild or harmless versions of all three viruses are surprisingly widespread. Investigations during the 1980s, in the rain forests of the Cameroons, found that 15% of the pygmies had antibodies in their blood to Ebola virus, implying they had been infected. No massive accompanying death toll was noted. Similarly, screening of large numbers of Africans from Nigeria and nearby nations, found many who showed signs of having been infected with Lassa, but who remembered only mild illnesses. Again, a very close relative of the Four Corners hantavirus has been found in Prospect Hill, Maryland. Prospect Hill hantavirus is carried by voles and while significant numbers of people have clearly been infected, it has never caused any noticeable illness, let alone death. This is in striking contrast to Four Corners hantavirus which causes 70-80% mortality.