THE APPROACH OF EVOLUTIONARY MEDICINE: LYING IN WAIT
The preceding examples share a common theme. High levels of harm-fulness are explained by the opportunities for hitchhiking from immobile individuals. Harmful pathogens can catch rides on mosquitoes, in water, on the hands of hospital attendants, or in the infected person when that person is transported from one cluster of susceptibles to another. There is yet another way in which pathogens can make it from a very sick infected host to an uninfected host: they can rely on the mobility of the not-yet-infected host. They can use a "sit and wait" strategy. If the pathogens are extremely durable in the external environment, they remain where they have been released and wait for the susceptible individuals to come to them. They may also contaminate clothing, blankets, or other possessions that are moved around after an owner's illness or death and lie there in wait for a new owner. Like pathogens transported by mosquitoes or water, durable pathogens can afford to convert more of their host into their own progeny because the immobility of a sick host is less costly to them.
This strategy is especially common among insect pathogens, which can stay viable outside hosts for years or even decades and then make soup out of an insect that happens by. But the strategy also explains why some pathogens of the human respiratory tract are so much more damaging than others. The mildest of human respiratory pathogens, such as the rhinoviruses that cause the common cold, lose their viability after a few hours outside the host. At the other extreme is the smallpox virus, the most lethal of human respiratory tract pathogens. No one knows exactly how long it can last in the external environment. In one study smallpox scabs were stored in an envelope that was left on a shelf in a lab cabinet. By sampling the scabs periodically, the researchers demonstrated viable viruses for thirteen years. They could not continue the study because thirteen years of testing had used up all the viruses in the envelope.
Imagine the number of people who might unwittingly enter a contaminated house or come into contact with contaminated materials over a thirteen-year period. Such durability explains why American Indians in the colonies of New York and Pennsylvania were decimated by smallpox. Lasting for a few days or weeks on the infamous smallpox-laden blankets distributed to American Indians from a colonial outpost would be difficult for most viruses, but not for a virus that could last more than thirteen years on a lab shelf. There were even more morbid consequences of this durability. In 1757, after French forces took over Fort William Henry in northeastern New York, their Indian allies began digging up English graves. They got the scalps they were after, but they also apparently retrieved smallpox viruses that were lying in wait in the corpses of those who had died from the disease.
Pathogens of the human respiratory tract tend to be only as bad as they are durable. The tuberculosis bacterium, for example, can last for weeks to months outside the human body. Before the days of the effective antituberculosis drugs isoniazid and rifampin, tuberculosis was second only to smallpox in its lethal effects on infected individuals. The next most deadly respiratory tract pathogens, those causing diphtheria, whooping cough, and pneumococcal pneumonia, range from being moderately to highly durable in the external environment, maintaining their viability for days to weeks. The mildest pathogens—those that cause little more than some nasal congestion, sneezing, or a cough— typically lose their viability within a few hours of exposure.