Staphylococcus is a genus of bacteria commonly known as staph. These bacteria are spherical and form clusters, which resemble grains of rice under a microscope, hence their name (from Greek words meaning "grain" and "to cluster"). Staphylococci can be found on the skin or in the noses of about 20%-30% of healthy people without causing any harm. However, some species of staphylococcus are pathogenic and can cause a wide range of illnesses including skin infections (such as impetigo), respiratory infections (such as pneumonia or sinusitis), and more severe systemic infections that can be life-threatening.