Hayflick is also known for his discovery of the cause of primary atypical pneumonia (“walking pneumonia”) in humans. The etiological agent was first thought to be a virus, but Hayflick showed that it was, in fact, a mycoplasma, a member of the smallest free-living class of microorganisms. The etiological agent was named by him as "Mycoplasma pneumoniae", and was first grown by Hayflick on a medium he developed and that bears his name. It is now used worldwide for mycoplasma isolation and research.