Every conceivable ecological niche on the planet is home to microbes and microbes have been around for hundreds of millions of years. Indeed, microbes may be the most abundant life form by mass, and they are highly adaptable to external forces. A vast majority of microbes are essential to human, animal, and plant life. Occasionally, however, a microbe that is identified as a pathogen can cause an acute infectious disease or triggers a pathway to chronic diseases, including some types of cancers.

Some disease-causing microbes, such as those that cause the flu and Lyme disease, spend part of their lives in insects or other animals before infecting the human body.

Other microbes live in soil and water, and are only harmful if we swallow them or absorb them through our skin. Soil microbes, for example, can enter the human body through a break in the skin or can be inhaled as dust.

A third class of microbes, such as HIV and tuberculosis, can exist in feces or mucus—but they can't exist for long outside a human host, so these microbes tend to remain in the human body for most or all of their life cycle.

Microbial threats continue to emerge, reemerge, and persist throughout human history. Some microbes cause newly recognized diseases in humans while others are previously known pathogens that are infecting new or larger population groups or spreading into new geographic areas.