The simplest life-forms on Earth have just one cell. An example of a one-celled life-form is a bacterium. These species of bacteria all cause diseases in humans. But many bacteria are not harmful, or even helpful.
Image of Streptococcus, a type (genus) of spherical bacteria that can colonize the throat and back of the mouth. Streptococci often occur in pairs or in chains, as shown here. Tina Carvalho, University of Hawaii at Manoa