Mosquitoes, ticks, fleas, lice, sandflies, tsetse flies, and triatomine bugs are the primary insects (and closely related arthropods) that spread diseases to humans. Together, these creatures transmit infections responsible for hundreds of thousands of deaths each year and sicken hundreds of millions more. Here’s what each one carries and how they pass it along.
Mosquitoes: The Deadliest Insect on Earth
No other insect comes close to the mosquito in terms of human harm. Three groups of mosquitoes cause the most trouble, and each tends to spread different diseases.
Aedes mosquitoes (especially Ae. aegypti) spread dengue, Zika, chikungunya, and yellow fever, all caused by viruses. Dengue alone is the most common viral infection transmitted by Aedes species worldwide. These mosquitoes bite during the day, prefer urban environments, and breed in small containers of standing water like flower pots and old tires.
Anopheles mosquitoes transmit malaria, a parasitic infection that killed an estimated 597,000 people worldwide in 2023, with roughly 263 million cases that year. That’s about 11 million more cases than the year before. Anopheles species bite primarily between dusk and dawn, and they’re found across tropical regions and parts of the continental United States.
Culex mosquitoes carry West Nile virus, Japanese encephalitis, and St. Louis encephalitis. West Nile is one of the most common mosquito-borne diseases in the continental U.S. Culex species tend to bite in the evening and nighttime hours and breed in ditches, storm drains, and other stagnant water sources.
All three groups also transmit lymphatic filariasis, a parasitic infection that causes severe swelling in the limbs and affects tens of millions of people in tropical countries.
Ticks and the Diseases They Carry
Ticks are technically arachnids, not insects, but they’re grouped with insect vectors because they transmit disease the same way: by feeding on blood. In the United States alone, ticks spread at least 15 distinct illnesses, including Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, anaplasmosis, babesiosis, and tularemia.
Lyme disease dominates the numbers. Approximately 476,000 people are diagnosed and treated for it each year in the U.S. It’s caused by bacteria transmitted through the bite of blacklegged ticks (also called deer ticks), and it produces a characteristic expanding rash in many cases, along with joint pain, fatigue, and neurological symptoms if untreated. Rocky Mountain spotted fever, ehrlichiosis, and tularemia also cause distinctive rashes that can help with early identification.
Globally, ticks transmit Crimean-Congo hemorrhagic fever, tick-borne encephalitis, and several forms of relapsing fever. Unlike mosquitoes, ticks don’t fly. They wait on grass and leaf litter, latch onto passing animals or people, and feed for hours to days, giving pathogens time to transfer.
Fleas: Plague and Beyond
Fleas are best known for spreading plague, the disease that killed millions during medieval pandemics. Plague still exists today. In the U.S., infected ground squirrel fleas are the most common source; globally, the Oriental rat flea is the primary carrier. The bacteria move from rodents to humans through flea bites.
Fleas also transmit murine typhus (spread by cat fleas and rat fleas or their droppings) and can pass along the bacteria responsible for cat scratch disease. Flea infestations are most common in homes with pets, and the insects thrive in warm, humid conditions.
Lice and Typhus
Body lice spread epidemic typhus and louse-borne relapsing fever, both bacterial infections. These diseases flourish in crowded, unsanitary conditions where people can’t regularly wash clothes or bathe, which is why outbreaks historically accompanied wars, famines, and displacement. Body lice live in clothing seams and move to the skin to feed, depositing infected feces near bite wounds. The person then unknowingly rubs the bacteria into the bite.
Sandflies and Leishmaniasis
Sandflies are tiny, often smaller than a grain of rice, and they transmit leishmaniasis, a parasitic disease that takes several forms. The skin form causes ulcers that can leave permanent scars. The visceral form attacks internal organs and is fatal without treatment. Sandflies also carry sandfly fever, a viral illness that causes high fever, headache, and muscle pain. They’re most active at dusk and dawn in tropical and subtropical regions, particularly in parts of South Asia, East Africa, the Middle East, and Latin America.
Tsetse Flies and Sleeping Sickness
Tsetse flies, found only in sub-Saharan Africa, transmit African trypanosomiasis, commonly called sleeping sickness. The parasite they inject through their bite eventually crosses into the brain, disrupting sleep cycles and causing confusion, personality changes, and, if untreated, death. Tsetse flies are large, aggressive daytime biters that live in woodland and savanna areas.
Triatomine Bugs and Chagas Disease
Triatomine bugs, sometimes called “kissing bugs,” spread Chagas disease across Latin America. Their transmission method is unusual. The bugs hide in cracks in walls and roofs of rural homes, emerge at night to feed on sleeping people, and typically bite exposed skin on the face. After feeding, they defecate near the bite. The parasite enters the body when the person rubs the bug’s droppings into the wound, their eyes, or their mouth.
Chagas disease can cause serious heart and digestive problems years or even decades after the initial infection. It’s primarily found in 21 Latin American countries, though cases increasingly appear elsewhere as people move between regions.
Flies That Carry Germs Mechanically
Not all disease-spreading insects inject pathogens through bites. Houseflies and similar species act as mechanical carriers. They land on feces or rotting material, pick up bacteria on their legs and mouthparts, then deposit those germs on food or surfaces. Houseflies can spread Shigella (which causes dysentery) and other diarrheal pathogens this way. Blackflies, a different group, transmit river blindness through their bites in parts of Africa and Latin America.
Why These Insects Are Spreading to New Areas
Rising temperatures are allowing disease-carrying insects to survive in regions where they previously couldn’t. Mosquitoes, ticks, and other vectors are shifting and expanding their geographic ranges as climates warm. Diseases not currently common in the U.S., including chikungunya, Chagas disease, and Rift Valley fever, are considered growing threats as their insect carriers move northward.
Protecting Yourself From Bites
The most effective protection against insect-borne disease is preventing bites in the first place. The EPA registers several active ingredients for skin-applied insect repellents. DEET is the most widely available, found in over 500 registered products. Picaridin, oil of lemon eucalyptus, and IR3535 are other proven options. For ticks specifically, treating clothing and gear with permethrin adds a layer of defense.
Beyond repellents, practical steps matter. Eliminating standing water around your home reduces mosquito breeding sites. Wearing long sleeves and pants in tick-heavy areas, checking your body after time outdoors, and keeping window screens in good repair all reduce your risk. In regions where triatomine bugs or sandflies are present, sleeping under insecticide-treated bed nets and sealing cracks in walls can make a significant difference.