Why cant the spread of an epidemic simplygrow indefinitely?
Answers
Explanation:
They are endemic. When there is a carrier as prevalent as mosquitos, then every time there are lots of mosquitos around (i.e. all year in the wet tropics, wet season in the wet-dry tropics, summer in the sub-tropics) then the disease spreads.
Diseases like malaria and dengue don't suddenly spread all over the world, because their vector (the mosquito) does not spread that rapidly. Mosquitos are small and don't travel fast or far. Instead, these diseases rage seasonally, or year round in their set locations. Malaria ravaged the poor and unhealthy of Rome every summer as mosquitos bred in the swamps; same with Georgia in the US. Dengue is a danger all year in the Congo or Liberia.
Occasionally a mosquito borne epidemic will move to a previously un-occupied region; this primarily happened during the long distance voyages of the Age of Exploration. But this is the exception rather than the rule. Because endemic mosquito born diseases stay in one place, the local population tends to have a higher resistance. Death toll is low and steady during each infectious season.
By comparision, fast-spreading and hard-hitting epidemics have different mechanisms for spreading. The worst epidemics are dominated by plague, influenza, smallpox, and cholera. Plague is spread by fleas via rats. Fleas don't travel long distances, but rats do. Influenza is airborne and transmitted by coughing and sneezing. Smallpox is spread through bodily fluids; usually tiny airborne droplets of mucus. Cholera is waterborne and spreads via the fecal-oral route: drinking poopy water. These four are the plagues that killed hundreds or thousands a day in historical big cities.