What are bacterial diseases? Bacterial diseases include any type of illness caused by bacteria. Bacteria are a type of microorganism, which are tiny forms of life that can only be seen with a microscope. Other types of microorganisms include viruses, some fungi, and some parasites. Millions of bacteria normally live on the skin, in the intestines, and on the genitalia. The vast majority of bacteria do not cause disease, and many bacteria are actually helpful and even necessary for good health. These bacteria are sometimes referred to as “good bacteria” or “healthy bacteria.” 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. Common pathogenic bacteria and the types of bacterial diseases they cause include: Escherichia coli and Salmonella cause food poisoning. Helicobacter pylori cause gastritis and ulcers. Neisseria gonorrhoeae causes the sexually transmitted disease gonorrhea. Neisseria meningitidis causes meningitis. Staphylococcus aureus causes a variety of infections in the body, including boils, cellulitis, abscesses, wound infections, toxic shock syndrome, pneumonia, and food poisoning. Streptococcal bacteria cause a variety of infections in the body, including pneumonia, meningitis, ear infections, and strep throat. Bacterial diseases are contagious and can result in many serious or life-threatening complications, such as blood poisoning (bacteremia), kidney failure, and toxic shock syndrome. Seek prompt medical care if you suspect that you have a bacterial disease. Seek immediate medical care (call 911) if you, or someone you are with, have critical symptoms of a bacterial disease, such as high fever, lethargy or unresponsiveness.