Human body lice used to kill more people than all of the bullets and swords and bombs combined in wartime. How could such a small insect do so much damage? They can spread a germ that causes typhus, a very serious human illness that produces a rash, fever, and death. The reason that typhus was so bad during wars is that people couldn't wash themselves and their clothes very often. If you clean yourself and your belongings, most of the eggs of the lice are likely to be removed. Cities that are being attacked by an enemy tend to neglect water systems and other services.
     What changed all of this? In the 1930's Swiss chemists discovered that an entirely new artificial chemical called DDT could kill lice and many other insects but be relatively harmless (at first, anyway) to humans. The US and its allies dusted the conquered people with DDT and this killed all the lice. Or so everyone thought.
     Actually, a tiny minority of the lice had a rare characteristic that allowed them to destroy the DDT as it entered their tiny bodies. This characteristic could be passed from parent to child louse through the genetic material in their eggs and sperm. The reason this ability was rare is that until DDT had been invented, it was an absolutely useless thing to be able to do!
     Naturally, when American troops started hosing everyone down with DDT, most lice died and typhus could not be easily spread. Water systems were restored and people were able to clean themselves properly again and the few lice that remained became even rarer. Nobody noticed that there were really still a few lice around.
     However, in subsequent years, earthquakes and other natural disasters produced war-like disruptions in cities. The health authorities naturally wanted to prevent an epidemic of typhus so they began to apply DDT. To everyone's amazement at the time, it was absolutely useless. Fortunately, at that time, certain antibiotics proved useful at killing the germs that actually caused typhus but, of course, not the lice. The population of the body lice had permanently changed because the DDT had killed all of the lice lacking the anti-DDT gene but none of those that had the protective factor. The human body lice species had permanently changed. This type of change brought about in the genetic make-up of an entire species of living things is called evolution.
     What good is any of this to you? As can be predicted from the above, many germ species have become resistant in their turn, to antibiotics. There are some species of germs and germ-carrying insects that no antibiotic or insecticide known to science will kill. It appears that our only real long-term protection for humans in this increasingly hostile world is the old-fashioned remedies that take diligence and hard work: thoughtfulness, cleanliness, and sanitation. Look around you for simple things you can do to prevent massive epidemics in the future: eat fresh food made in clean surroundings, wash and comb your hair every day, look for circumstances around  the home that can encourage pestilence such as standing water (encourages disease-carrying mosquitoes) and trash (home for rats and mice).
     Survival of our species through the coming plagues favors those who are prepared to work.