Also called rubeola, measles spreads easily and can be serious and even fatal for small children. While death rates have been falling worldwide as more children receive the measles vaccine, the disease still kills more than 200,000 people a year, mostly children.

As a result of high vaccination rates in general, measles hasn't been widespread in the United States in about two decades. Most recent measles cases in the U.S. originated outside the country and occurred in people who were unvaccinated or who didn't know whether or not they had been vaccinated.

Pathogens are different and can cause disease upon entering the body. All a pathogen needs to thrive and survive is a host. Once the pathogen sets itself up in a host's body, it manages to avoid the body's immune responses and uses the body's resources to replicate before exiting and spreading to a new host.