This is heard very commonly when someone sees an unwanted animal, especially near food. Flies, roaches, pigeons, rats, etc. are all claimed to be filthy and disease ridden, warranting their extermination from our homes. How realistic is this? Are these and other animals actually likely to transfer illnesses, or is their chance of being contaminated with harmful microbes just the same as your own body? I don't see any reason why a fly or rat's feet should have more or more harmful bacteria on them than my own hands. Are they inherently carriers of human-harmful illness?
Some "pests" are more likely to transfer disease.
Flies frequently breed in and around feces, and carry small amounts of it on their feet. When they land on our food or eating/cooking surfaces, they can leave contaminants:
Houseflies spread germs in other ways, too. The trouble is-- houseflies breed in and around manure piles (manure is the big, wet, warm droppings of cows, sheep, horses, and other large mammals), garbage, and rotting flesh. All of these places provide a good source of food for the maggots when they hatch. Flies have sticky pads on their feet, and every time a fly lands on something in our home and walks around on it, it leaves behind little bits of manure, garbage, or rotting flesh. When they walk on our food or our countertops, they leave behind germs from the last place they visited.
Beyond that, they are vectors for other infectious diseases:
Flies can spread typhoid fever. During the Spanish-American War in 1898, typhoid was spread by flies and killed over 5,000 soldiers. The battles themselves in this war only killed 4,000 soldiers! Flies also spread malaria, yellow fever, sleeping sickness, and dysentery.
Mice are also carriers of some pretty nasty diseases. The Hantavirus is carried by mice (primarily deer mice), but does not make the animal sick. Humans can catch the virus by exposure to the dust of dried mouse droppings, and it can be fatal.
Pigeons can cause some minor health problems.
Three human diseases are known to be associated with pigeon droppings: histoplasmosis, cryptococcosis, and psittacosis.
Histoplasmosis Histoplasmosis is a disease caused by a fungus, which grows in pigeon droppings. It also grows in soils and is found throughout the world. When cleaning droppings a person may breathe in some of the fungus, which in cases of high exposure can cause infection. Common activities, such as cleaning off windowsills, will not result in high exposures.
Symptoms of histoplasmosis begin to appear about 10 days after initial infection and include fatigue, fever, and chest pains. Most people, however, do not show any symptoms. Those with compromised immune systems such as cancer patients or people living with HIV/AIDS are generally more at risk of developing histoplasmosis. The disease cannot be transmitted from person to person.
Cryptococcosis Cryptococcosis is another fungal disease associated with pigeon droppings and also grows in soils throughout the world. It is very unlikely that healthy people will become infected even at high levels of exposure. A major risk factor for infection is a compromised immune system. According to the US Centers for Disease Control (CDC), nearly 85 percent of cryptococcosis patients are HIV-positive.
Psittacosis Psittacosis (also known as ornithosis or parrot fever) is a rare infectious disease that mainly affects parrots and parrot-like birds such as cockatiels, and parakeets, but may also affect other birds, such as pigeons. When bird droppings dry and become airborne people may inhale them and get sick.
In humans, this bacterial disease is characterized by: fatigue, fever, headache, rash, chills, and sometimes pneumonia. Symptoms develop about 10 days after exposure. Psittacosis can be treated with a common antibiotic.
Since 1996, fewer than 50 confirmed cases were reported in the United States annually. In New York City, psittacosis is very rare with less than one human case identified each year. According to the CDC, about 70% of infected people had contact with infected pet birds. Those at greatest risk include bird owners, pet shop employees, veterinarians, and people with compromised immune systems. No person-to-person cases have ever been reported.
In addition to the Bubonic Plague Darwy mentioned, rats can spread a number of diseases through the fleas that frequently infest them.
Since the early 1990s, more than 20 species of Bartonella bacteria have been discovered. They are considered to be emerging zoonotic pathogens, because they can cause serious illness in humans worldwide from heart disease to infection of the spleen and nervous system.
Scientists have found that rodents carry several pathogenic species of Bartonella, such as B. elizabethae, which can cause endocarditis and B. grahamii, which was found to cause neuroretinitis in humans. Although scientists are unsure about the main route of transmission, these infections are most likely to be spread by fleas. Ctenophthalmus nobilis, a flea that lives on bank voles, was shown to transmit different species of Bartonella bacteria. These pathogens have also been found in fleas that live on gerbils, cotton rats and brown rats.
Rats can also spread diseases to humans through their urine which can be transmitted by contact with water or moist plants and soil. The organisms in the urine can enter the human body through cuts or abrasions.
Salmonellosis can also be spread by drinking contaminated food or drink that had come into contact with rat feces or urine.
TLDR version: Yes, some pests are nasty, filthy disease carriers that can cause serious illness.