The rampant spread of disease was common in the days before penicillin and other "wonder drugs" of the twentieth century. Our ancestors lived in fear of epidemics, and many of them died as the result of simple diseases that could be cured today with an injection or a prescription.
If you ever wondered why a large number of your ancestors disappeared during a certain period in history, you may want to investigate the possibility of an epidemic. Many cases of people disappearing from records can be traced to dying during an epidemic or moving away from the affected area.
Some of the epidemic statistics are staggering. For instance, the influenza epidemic of 1918 and 1919 killed more people than did World War I. Any major outbreak of disease was accelerated by a total absence of sanitary procedures and lack of knowledge. In Europe during the Middle Ages, the homes of the citizens often had roofs and walls made of straw, floors of dirt, and dwellings where animals were kept inside. The city streets, if that's what you could call them, often were barely wide enough for a single cart to pass, and they were perpetually covered with mud, garbage, and excrement. For lack of heated water, people rarely bathed, and fleas were commonplace. It is a wonder that anyone survived under these conditions!