Millions of bacteria normally live on the skin, in the intestines, and on the genitalia. Many human infections are caused by either bacteria or viruses. A bacterial infection is a proliferation of a harmful strain of bacteria on or inside the body. Infectious bacteria can make you ill. They reproduce quickly in your body. Many give off chemicals called toxins, which can damage tissue and make you sick. Bacteria can infect any area of the body. Pneumonia, meningitis, and food poisoning are just a few illnesses that may be caused by harmful bacteria. Bacterial skin infections are usually caused by gram-positive strains of Staphylococcus and Streptococcus or other organisms. Harmful bacteria that cause bacterial infections and disease are called pathogenic bacteria. Bacterial diseases occur when pathogenic bacteria get into the body and begin to reproduce and crowd out healthy bacteria or to grow in tissues that are normally sterile. Harmful bacteria may also emit toxins that damage the body.