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Chapter 3. Transmission, overcrowding, a... > Many diseases become milder with tim... - Pg. 60

40 germs, genes, & civilization 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 circum- stances, they can spread from person to person. Other dis- eases 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 inca- pacitates the first victim too quickly, it will undermine its own transmission. Consider, too, the spread of the virus from vil- lage 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 popu- lation 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 stag- gers 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