Lynn, a doctor could explain it better, but here's my historical researcher's take on the subject -- People who grew up and lived in comparatively isolated rural areas had little or no exposure to various diseases, therefore had no opportunity for their bodies to develop antibodies to counteract those diseases.

People who lived in crowded urban areas were exposed to all sorts of diseases and, whether or not they contracted the disease, mere exposure caused the development of antibodies.

The classic extreme example was the exposure of the Indians in North and South America to European diseases in the sixteenth and seventeenth centuries. Having no immunity to these new diseases, hundreds of thousands died.

On a smaller scale, thousands of Confederate and Union soldiers died of such things as measles, smallpox, typhoid and other diseases, despite the fact that they were healthy young men in the prime of life. Those who survived the disease, or survived exposure without developing the disease, were immune from that point. As a result, it was during the Civil War that the U.S. Army (and, to a much lesser extent, the Confederate army) began systematically immunizing soldiers against smallpox, recognizing that giving a soldier a few pin-pricks with an infected needle would provide enough exposure to prompt the body to produce antibodies, but not enough (in most cases) to produce a full-blown case of the disease.