Streptococcus pyogenes, a spherical bacteria that typically grows in long chains, can cause minor infections like impetigo to potentially deadly diseases like streptococcal toxic shock syndrome. The cells possess a number of defense mechanisms, known as virulence factors, which allow them to evade the host's immune system and spread through its tissues.

In this image, human neutrophils--white blood cells that are one of the body's first lines of defense--are engulfing S. pyogenes cells through a process known as phagocytosis.