From an immunological perspective, an epidemic may change a human population dramatically. Those individuals who have contracted a disease and recovered will likely be immune to reinfection because they harbor vastly increased concentrations of the lympho- cytes that make the antibodies that are most destructive of that partic- ular pathogen. Adult immunity to childhood diseases such as mumps depends not on changing human gene pools but on changing the con- centrations of different kinds of antibodies within each individual.
Small size gives our pathogens another advantage: their enormous numbers. Each of us carries around (mostly in our digestive and res- piratory systems) more bacterial cells than there are people on Earth. These enormous numbers mean that even improbable sorts of muta- tions will occur with appreciable frequency and that any mutant bac- terial strain with even the most minute advantage over the others will soon prevail numerically. We can expect our pathogens’ quantitative characteristics to evolve rapidly to whatever values are optimal for present circumstances.
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