text: Some bacteria are harmless. Your skin, mouth, bowels and other places are filled with them. Many are even necessary to your survival because they help digest food or keep infectious organisms at bay. Cows could not digest grass without bacteria in their stomachs. The troublemakers are the ones that are able to make harmful chemicals that act like acids or poisons. Botulism, for instance, is caused by a bacterial poison. Pus, for the most part, is the body's response to illness. It's a collection of white cells that the body recruits for an area of infection. At other times, bacteria are so irritating to your system that your own body's defenses go overboard and attack not only the bacteria but also the tissues that host them.\n\nHow do I know I have a bacterial infection?\n\nBacterial infection usually makes you noticeably sick, whether it's an infected ingrown toenail or bloody diarrhea. Sometimes, however, they produce very little awareness but still can go on to cause immense trouble. Sexually transmitted disease, for instance, can be very subtle and can leave you infertile before you know what hit you. Even if an infection is obvious, it often takes a modern laboratory to identify the exact germ and, more important, tell the doctor what to use to treat it. Most bacteria can be grown in the laboratory and tested for their reaction to different antibiotics. This is why the doctor wants to collect a specimen from your infection, wherever it is.\n\nMany infections produce inflammation, a process generated mostly by your immune system, which appears as redness from an increased blood supply, swelling from fluid accumulating in the tissues, pain from nerve irritation, and heat, both localized warmth and generalized fever.