Lyme disease is spread by deer ticks like this one. A study finds that some people can be reinfected many times with the bacteria that cause the disease. Lauree Feldman/Getty Creative Images hide caption

itoggle caption Lauree Feldman/Getty Creative Images

Bacteroides species are some of the most common bacteria in the human gut. Enviornmental Health Perspectives hide caption

itoggle caption Enviornmental Health Perspectives