The Filthy Housefly: Spreading Illness and Disease

Close-up view of a housefly.

Anand Joshi / Moment / Getty Images

As their name indicates, houseflies are regularly found around the home. Because they breed and feed on human food and waste, they are generally found about anywhere that humans are. The common housefly is often called the "filth fly" because it can contaminate food and surfaces every time it lands. If a fly sets itself down on the edge of your glass, it may have just flown over from a pile of trash, manure, or other waste.

Even the U.S. Department of Agriculture's Agricultural Research Service (USDA/ARS) description shows how filthy and disease-ridden this common pest can be:

"It's common knowledge that house flies are carriers of disease. That's why there's such widespread effort to keep them out of our kitchens and away from our food."

Identifying Houseflies

Although there are other large flies, the common housefly can be identified by a few distinguishing features:

  • Houseflies are about 1/4-inch long.
  • They are grayish.
  • They have broad, dark stripes down the mid-section of their bodies.
  • Their most dominant feature is their large, reddish, compound eyes.

In addition to these physical characteristics, houseflies can be identified by something they don't do: bite. The housefly does not bite because it can't. It has "sponging" mouthparts, so it can only suck up liquids. If you are bitten by a large fly, it is most likely a horsefly, deerfly, stable fly, or blackfly.

Why Can't You Swat That Fly?

It is the fly's compound eyes, tiny body hairs, and speed that make it so difficult to swat. The eyes and hairs enable it to sense changes in airflow so that well before your flyswatter gets close, the fly has flown off. This, along with its ability to fly at speeds of 5 mph—with bursts of up to 15 mph when threatened—gives this fly the power to be well away before your swatter, hand, or the newspaper can land.

Housefly Habits and Behavior

In addition to feeding on garbage, the housefly lays its eggs on garbage, animal feces, and other decaying organic material, such as lawn clippings. The eggs hatch within a day and the emerging maggots (larvae) feed on the waste to develop into adult flies in less than a week. A fly can live about a month, and during this time, the female fly can lay about 2,000 eggs in batches of 75 to 100.

The housefly can travel up to 20 miles, but it generally stays within two miles of where it hatched. Most common in spring and summer, these flies often enter homes through open doors and windows, but they can also get in through cracks around doors and windows, torn screening, and similar small openings.

Why Houseflies Are a Problem

  • Houseflies spit on your food. The housefly can only digest liquid food, so when it lands on your sandwich, it immediately regurgitates saliva onto it and then sucks in the liquid this creates (or most of it anyway). Some of that saliva is likely to still be on the bread when you go to take your next bite.
  • Houseflies leave specks. Those little black dots that you see in places where flies have spent a bit of time are their feces.
  • Houseflies fly from the trash to the table. Flies care little whether they feast on food from the dumpster outside or your table, and they will fly back and forth between the two with the ability to carry more than a million bacteria on their feet and body.
  • Houseflies can carry disease. According to university reports, including the University of Rhode Island and Penn State, at least 65 diseases have been associated with houseflies. The bacteria and viruses they carry can cause food poisoning, diarrhea, eye infections, dysentery, cholera, and even tuberculosis.