3. Transmission, overcrowding, and virulence

Virulence and the spread of disease

How a disease spreads greatly affects its impact on human society. Diseases that spread efficiently will clearly infect more people. Less obvious, but no less important, its transfer mechanism determines how virulent a disease may become.

We must tackle two widespread misconceptions. Both generalizations are half true, and scientific investigations have only recently discovered their flaws. The first is the idea that because diseases adapt to their hosts, they will inevitably become milder if we just wait long enough. Thus, syphilis was extremely virulent when first introduced into Europe but nowadays is much milder. Similarly, childhood diseases such as measles ...

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