Others are far more specialized and can survive in only one type of environment. Deadly and widespread as it is, HIV cannot exist for long outside the human body, for example. Some disease-causing microbes enter the human body and stay there for part or all of their life cycle. When pathogenic microbes spend part of their lives in insects or other animals before they move to the human body, they are called vector-borne agents. Some microbes that live in water are harmful if swallowed, or if they penetrate the skin.
Soil microbes can enter the human body through a break in the skin or can be inhaled as dust.
Humankind's initial method for survival was to hunt and gather food. Later, people developed agricultural societies, which sustain much higher human population densities than the hunter-gathering lifestyle. Jared Diamond, in his book Guns, Germs, and Steel, traces the history of how people's advancement in civilization and agriculture has profoundly affected how germs became closely associated with infectious diseases. Whereas hunters frequently are on the move, farmers are sedentary and live close to their own sewage, thus providing microbes a short path from one person's body into another's drinking water. Some farming populations make it even easier for their own fecal bacteria and worms to infect new victims, by gathering their feces and urine and spreading them as fertilizer on the fields where people work.