How Infectious Disease Spreads
The prevalence of infectious disease in history has fluctuated wildly. David Clark outlines the different ways a disease can spread and examines how many diseases become milder over time.
Different contagious diseases spread in different ways. We can subdivide these into three major mechanisms. Some diseases spread by direct person-to-person contact. Others spread indirectly via inanimate objects. Yet a third strategy is for insects or other intermediaries to carry the infectious agent. The way an infection spreads greatly affects whether it becomes milder over the ages, stays much the same, or gets more virulent.
Certain diseases require prolonged contact of an intimate nature to move from one person to another. These diseases are relatively hard to catch and can often be avoided by changing personal behavior. The sexually transmitted diseases (STDs) such as syphilis, AIDS, and gonorrhea illustrate this scenario. Strictly speaking, the transfer of body fluids is involved here. This is important from a practical viewpoint, because such infections can also be spread by improperly sterilized hypodermic needles. This occurs both among the intravenous drug users of the industrial world and in the clinics of Third World nations that lack money for disposable syringes.
Other diseases are spread by direct personal contact, but with less intimacy than for STDs. Many venereal diseases probably evolved from ancestors who infested the skin and body surface in a more general way. For example, the chlamydia that infect the genitalia are closely related to those causing the eye infection trachoma. The specialized sexual versions likely arose in historical times only as human populations became denser.
Some germs are transmitted by bodily contact or via nonliving objects such as doorknobs, paper money, clothes, and bed linen. Highly contagious virus diseases such as colds, influenza, measles, and smallpox are typical of this group, although most of these can also be transferred through the air. Many infections are transmitted from person to person through the air by coughing or sneezing. This is known as droplet transmission, because the germs are carried in microscopic droplets of saliva, phlegm, or mucus. Many of these germs fail to survive if they dry out completely. Tuberculosis, influenza, and colds are familiar examples. As the nursery rhyme says:
I sneezed a sneeze into the air,
It came to ground I know not where.
But hard and cold were the looks of those,
In whose vicinity I snoze.
Infectious agents can also be taken in with food or drink. Poor hygiene may result in food or drinking water being contaminated with human or animal waste. Typically, such infections affect the gastrointestinal tract and include the many types of protozoa, bacteria, and viruses that number diarrhea among their symptoms. The purpose of diarrhea, from the germ's viewpoint, is to provide an exit mechanism from the body and to recontaminate the water supply. Examples of waterborne diseases include Cryptosporidium (a protozoan), cholera (a bacterium), and polio (a virus). Infections caught from food are often referred to as “food poisoning,” despite resulting from bacteria or viruses instead of poisonous chemicals.
Diseases are often carried by insects such as mosquitoes and flies or by animals such as rats and mice. These are referred to as vectors. Sometimes multiple vectors are involved, such as in the spread of the Black Death by fleas carried by rats or typhus fever by ticks carried by rodents. Controlling vectors usually limits the spread of a disease far more effectively than treating infected humans. Insects and their relatives, the ticks and mites, are the most common vectors. However, other animals may act as vectors, as in the spread of rabies by bats and squirrels, or of West Nile virus by migrating birds. Plague and typhus normally rely on fleas and ticks to distribute them, although, under some circumstances, they can spread from person to person. Other diseases are obliged to spend part of their life cycles in a second host. Thus, malaria must pass from human to mosquito and back again to complete its developmental cycle.
Many diseases become milder with time
Let's consider the spread of a virulent virus like Ebola from the viewpoint of the virus. After infection, the victim will most likely die in a few days. Before the first victim dies, the virus must find another victim to infect. Clearly, the longer the first victim moves around, the greater the chances are of the virus making contact with someone else. If the virus incapacitates the first victim too quickly, it will undermine its own transmission. Consider, too, the spread of the virus from village to village. As long as the virus stays in the same village, where plenty of potential victims live close together, it can get away with killing fast. But what happens when the village has been wiped out? The virus must now find another population center. This requires an infected person who is still fit enough to travel. Over the long term, movement between population centers may matter more than how a disease spreads locally within a group of people.
Now consider two slightly different Ebolaviruses. One kills in a day or two. The second takes a whole week. Virus 1 may wipe out a whole village, 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 effective, and many small villages escaped entirely from epidemics that decimated nearby towns.
By comparison, a less virulent Ebolavirus will spread much more effectively. Infected refugees fleeing an infected village may 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, will predominate. Many diseases appear to have done just this and have evolved to become milder. Examples include gonorrhea and syphilis (caused by bacteria), and measles, mumps, and influenza (caused by viruses). What unites these diseases is that all are transmitted directly from person to person.
Ebolavirus infects humans now and then after emerging from some animal host, probably bats. It wipes out a few people in close contact, and then the mini epidemic burns itself out. Much the same is true of Lassa fever and other highly virulent diseases that burst out of the jungle every so often. Although they give the press the opportunity to spur apocalyptic hand-wringing, they are unlikely to spread far without getting milder.