7 Dangerous Flies You Should Definitely Avoid

Most of us instinctively dislike flies—and for good reason. While some are harmless, certain species of flies are more than just annoying; they can bite, spread diseases, and pose serious health risks. So, what exactly makes these tiny insects so dangerous?

Let’s dive into seven species of flies that you’ll want to steer clear of, especially if you’re spending time outdoors in warm, humid environments.

Black Flies: Tiny But Troublesome

Black flies are small, dark insects with arched backs, short antennae, and broad, fan-like wings. They tend to swarm during late spring and early summer, especially in warm, humid weather. Only the females bite—because they need blood to develop their eggs—while the males stick to nectar.

In the U.S., black fly bites are usually harmless unless someone has a serious allergic reaction. Symptoms can include swelling, fever, nausea, and swollen lymph nodes, a condition sometimes called «black fly fever.» However, in parts of Africa and South America, black flies can transmit a serious disease called river blindness, making them far more dangerous.

Black flies typically target the head, neck, and face. Their bites are painful and can lead to itchy welts and swelling.

Deer Flies: Painful Bites and Potential Illness

Also known as deer bloodsuckers or moose flies, deer flies are medium-sized with large heads and distinctive dark bands on their wings. These flies don’t just bite—they slice through the skin with razor-sharp mouthparts to get to the blood.

Their bites hurt and often leave behind red bumps or even scars. Common across the U.S., especially in the Southwest, deer flies hang around lakes, marshes, and wetlands where they lay eggs. They’re attracted to movement, shiny surfaces, carbon dioxide, and body heat, making people and animals prime targets.

The real danger? Deer flies can transmit tularemia (also known as rabbit fever), a bacterial infection that causes fever, skin ulcers, and swollen lymph nodes. It’s treatable with antibiotics, but without proper care, it can be life-threatening. Some people may also have allergic reactions to their saliva, leading to hives or wheezing.

Tip: When in deer fly territory, wear long sleeves and use insect repellent to stay protected.

Horse Flies: Fast, Persistent, and Painful

As the name suggests, horse flies are known to torment horses—and anyone nearby. Large and dark-colored, these flies are strong fliers that can travel over 30 miles in search of a host. They thrive in warm, humid areas near water and are active during the day, especially in summer.

Horse fly bites are incredibly painful due to their scissor-like mouthparts, which cut into the skin. While they don’t typically transmit disease to humans, they can infect livestock by introducing bacteria and viruses through their bites.

In horses, horse flies are known to transmit equine infectious anemia (also called swamp fever), which can be deadly. Even without symptoms, infected animals can still spread the disease, making containment difficult. Other livestock may experience slowed growth and reduced milk production due to stress from the bites.

While it’s nearly impossible to eliminate horse flies completely, repellents, traps, and protective gear can help reduce their numbers.

Robber Flies: Nature’s Fierce Predators

With their large, hairy bodies and piercing mouthparts, robber flies (also known as assassin flies) look like miniature monsters. These aggressive predators mostly feed on other insects, often catching them mid-air.

While robber flies generally leave humans alone, they can deliver a painful bite if provoked. The good news? They help control pest populations in your garden by eating wasps, grasshoppers, beetles, and other harmful insects.

So unless you mess with one directly, they’re more friend than foe.

Sand Flies: Small But Seriously Dangerous

Sand flies are tiny, fuzzy insects with long legs and golden, brown, or grayish coloring. Despite their size—no bigger than a mosquito—they pack a painful punch.

Unlike many biting flies, sand flies are active during dusk and sometimes in daylight. They thrive in tropical and subtropical regions and breed in moist, organic-rich environments. Their long mouthparts allow them to pierce the skin and draw blood with ease.

Sand fly bites often leave itchy welts, blisters, or red bumps—particularly on exposed areas like your ears, arms, and feet. In severe cases, they can transmit leishmaniasis, a parasitic disease that causes skin ulcers and, if untreated, can leave long-lasting scars.

Tsetse Flies: The Most Dangerous Fly on the Planet

Tiny but lethal, tsetse flies are arguably the most dangerous flies in the world. Found primarily in sub-Saharan Africa, they inhabit areas near rivers, lakes, and tropical forests.

These flies are most active during the hottest part of the day. Interestingly, males often target humans, while females prefer large animals.

What makes tsetse flies so dangerous is their ability to transmit African trypanosomiasis, also known as sleeping sickness. This serious illness causes fever, fatigue, and swollen lymph nodes, eventually affecting the nervous system. Without treatment, it can be fatal.

Each year, sleeping sickness claims up to 300,000 lives, and livestock are also at risk. In animals, the disease—known as nagana—can lead to weight loss, reduced fertility, and death.

Yellow Flies: Silent But Painful Attackers

Yellow flies (Diachlorus ferrugatus) are similar in size and behavior to deer flies. With their small, yellow abdomens and hairy bodies, they may not look menacing—but don’t let their size fool you.

These flies are most active in hot summer months and early fall, especially during daylight. What sets them apart is their tendency to bite indoors, often targeting unsuspecting humans and pets—particularly dogs.

Their bites are painful and often go unnoticed until the discomfort sets in. Since they’re silent fliers, you may not realize a yellow fly is nearby until it’s too late.

Final Thoughts

Not all flies are created equal. While some are merely annoying, others can pose serious health risks to both humans and animals. Knowing which species to avoid—and how to protect yourself—can help you enjoy the great outdoors with fewer itchy regrets.

So next time you swat at a buzzing visitor, it might be worth checking which kind of fly it is. It could be more than just a nuisance.