In general, Rats can spread disease through biting or contact with their fecal matter, or droppings. Don't approach a wild rat - generally, they're more afraid of you than you are from them. Rats will bite or scratch if frightened or handled, so leave them alone. Roof rats have long been named as carriers of the fleas responsible for the plague (also known as the Black Death) in the Middle Ages. While this plague is no longer as serious a threat to humans, roof rats are still potential carriers of disease.
Rat bites may be contaminated with Streptobacillus moniliformis or Spirillum minus. S. moniliformis is most common in the US. These infections may lead to Rat Bite Fever. Symptoms of Rat Bite Fever may appear as much as 10 days after the bite, and are likely to happen after the wound itself is already healed. Watch for fever, headache, vomiting, and pain in the back and joints. Two to 4 days after the onset of fever, a rash may occur on the hands and feet, and one or more large joints may become swollen, red, and painful. If the victim exhibits any of these symptoms, see a doctor immediately.