Streptococcus is a bacterium and not a virus, according to WebMD. There are several different types of streptococcal bacteria, and some cause more serious illnesses than others.
MedlinePlus indicates that there are two types of streptococcal bacteria: group A and group B. Group A streptococcal bacteria cause strep throat, scarlet fever, impetigo and cellulitis, while group B streptococcal bacteria can cause blood infections, pneumonia and other serious infections in adults. Streptococcus pyrogenes, the cause of strep throat, is easily treated with antibiotics, notes WebMD. Approximately 20 percent of the population harbor these bacteria during the winter without exhibiting any symptoms of illness.