The Bible mentions ten plagues in Pharaonic Egypt. Nine are common occurrences in the Middle East, like locusts and gnats. But the tenth one was a selective pandemic that killed Egyptian children and passed over the Jewish population. How could that happen?
Easily. The Jews had suffered repeated epidemics, while the Egyptians were virgin and specially the children had no experience with the disease. What disease was that? Smallpox, for example. According to historical accounts, age-specific smallpox mortality was gleaned from the age distribution of smallpox burials in York (England) between 1770 and 1812. Smallpox preferentially infects young, pre-reproductive members of the population since they are the only individuals who are not immunized or dead from past infection. Because smallpox preferentially kills pre-reproductive members of a population, it generates stronger selective pressure than plague. Unlike plague, smallpox does not have an animal reservoir and is only transmitted from human to human. Smallpox could have exerted a selective force sufficient to reach 10%. Source: Wiki.
Pic.: A child mummy with smallpox.