Public Health

Vector-Borne Diseases

As with any trip into the Sierra Nevada, it's possible (though unlikely) while visiting Yosemite that you could be exposed to a variety of vector-borne diseases. Here are some general tips to reduce your risk:

  • Avoid contact with mosquitoes and ticks.
    Many mosquitoes are most active at dusk and dawn, mainly during the summer. Be sure to use insect repellent and wear long sleeves and pants or consider staying indoors during these hours. Ticks cling to plants, waiting for a host (you) to walk by; walk down the middle of trails and avoid areas with high grass and leaf litter.
  • Use an effective insect repellent.
    Repellents containing DEET, picaridin, oil of lemon eucalyptus, IR3535, para-menthane-diol, or 2-undecanone typically provide reasonably long-lasting protection against mosquitoes, according to the Centers for Disease Control and Prevention. (Search for a repellent that is right for you.)
  • Find and remove ticks from your body
    Soon after going indoors, bathe or shower, then conduct a full-body tick check using a mirror. Examine your children, too, and also check pets, backpacks, clothing, etc.
  • Avoid sleeping in rodent-infested areas or near animal burrows. If you see evidence of rodent activity in your room or other facility, contact park staff (don't clean it up yourself).
  • Avoid contact with wildlife and keep food and trash stored properly.
    Wild animals in Yosemite can transmit numerous diseases, including plague, rabies, and hantavirus. Keeping your distance and your food from wildlife not only protects them, it also protects you from injury and exposure to diseases.

    If you encounter a mammal, particularly a raccoon, skunk, fox, coyote, or bat, that is behaving erratically, don’t touch the animal. Instead, report the sick animal to a park employee.

    In addition to keeping bears away, storing your food properly also reduces your exposure to rodents and their fleas, which may carry plague.

Hantavirus Pulmonary Syndrome

Hantavirus Pulmonary Syndrome (HPS) is a severe, sometimes fatal, respiratory disease in humans caused by infection with a hantavirus. California Department of Public Health and Centers for Disease Control and Prevention websites contain important information that can help you recognize and and reduce your risk of contracting of Hauntavirus.


Plague is a highly infectious bacterial disease primarily affecting rodents. Humans and other animals can get plague if they visit or live in areas where wild rodents are sick or dying from plague. Plague has been identified throughout the Sierra Nevada including Yosemite National Park. Wild rodents (including squirrels and chipmunks) are the principal source of plague in California. California Department of Public Health and Centers for Disease Control and Prevention websites contain important information that can help you recognize and and reduce your risk of contracting of plague.

West Nile Virus

West Nile virus (WNV) is a potentially serious illness. Experts believe WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall. Most often, WNV is spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite. California Department of Public Health and Centers for Disease Control and Prevention websites contain important information that can help you recognize and reduce your risk of contracting West Nile virus.

Tick-Borne Relapsing Fever

Tick-borne relapsing fever (TBRF) is a rare but serious illness in people caused by bacteria that are carried by soft ticks. TBRF typically causes flu-like symptoms that disappear quickly after a few days. This is followed by a return (or relapse) of symptoms a few days later. This cycle can repeat for weeks if not treated. Soft ticks look and behave differently than the hard ticks ("dog ticks" or "deer ticks," for example) commonly found while hiking in many parts of California. Soft ticks aren't found along hiking trails but typically live in dark, cool places such as rodent nests, and shaded wood piles outside of buildings. Soft ticks are found mainly in forested foothill and mountain regions above 3,000 feet, including Yosemite National Park. Those who get sick with TBRF are usually infected while visiting mountain areas and staying in cabins or buildings that are infested with rodents and soft ticks. People can protect themselves from TBRF by keeping rodents out of buildings. The California Department of Public Health and Centers for Disease Control and Prevention websites contain important information about tick-borne relapsing fever. If you are diagnosed as having relapsing fever, and you believe you contracted it in Yosemite, have your doctor contact the park's environmental health officer/public health consultant at 209/379-1209.

Lyme Disease

If you are bitten by a tick, and later experience flu-like symptoms, contact your doctor and mention you had a tick bite. The California Department of Public Health and Centers for Disease Control and Prevention websites contain important information about Lyme disease. If you are diagnosed as having Lyme disease, and you believe you got it in Yosemite, have your doctor contact the park sanitarian at 209/379-1209.


Rabies is a viral disease that affects the central nervous system. The virus is usually passed to humans via the bite of a rabid animal. Occasionally, rabies can be transmitted if the saliva of an infected animal gets into a fresh scratch, break in the skin, or contact with mucous membranes (eyes, mouth, nose). Throughout California, most cases of rabies occur in skunks and bats.

Always enjoy wild animals from a distance and do not do not handle, feed, or attract wild animals. Yosemite has an ecologically rich population of bats. If you see a bat that is behaving erratically, is unafraid of humans, or is lying on the ground, it may be sick. It is important not to touch or approach bats closely.

Rabies is 100% preventable if appropriate medical attention is given prior to the development of symptoms, but is 100% fatal if an exposure is not treated. The California Department of Public Health and Centers for Disease Control and Prevention websites have important information about rabies.


Other Public Health Issues in Yosemite

Harmful Algal Blooms

Harmful algal blooms (HABs) are blooms of algae or cyanobacteria that produce toxic compounds that threaten public health and the ecosystem. Threats from HABs may be more pronounced during periods with low or stagnant water flow, high-intensity sunlight, and elevated temperatures—conditions experienced during drought and which may be increasingly common in the future under climate change.

According to the CDC, people can become sick when exposed via skin contact through activities like swimming, breathing in tiny airborne droplets or mist that contain toxins, swallowing water that contains toxins, or eating food or supplements containing toxins. Symptoms depend on the cyanotoxin involved but include stomach pain, headache, vomiting, diarrhea, liver damage, and neurological symptoms such as muscle weakness or dizziness. Animals can become sick when they drink water containing algal or cyanobacterial toxins, swim or play in water containing algal or cyanobacterial toxins, eat or lick toxic algae or cyanobacteria that is in the water, on the shore, or on their fur. Animals can also become exposed through eating fish or other dead animals that contain algal or cyanobacterial toxins, and may experience illness symptoms such as excessive salivation, vomiting, fatigue, staggered walking, difficulty breathing, convulsions, liver failure, or death within hours or days of exposure.

The HAB Incidents Reports Map shows blooms in Yosemite and elsewhere in California.

Please report any large algae blooms. Include where the bloom was occurring, the date you saw it, and a general description.


Norovirus—the stomach bug (a gastrointestinal illness—is a highly contagious virus. Norovirus infection causes gastroenteritis (inflammation of the stomach and intestines). This leads to diarrhea, vomiting, and stomach pain.

Norovirus illness is often called by other names, such as food poisoning and stomach flu. While noroviruses can cause food poisoning, other germs and chemicals can also cause food poisoning. Norovirus illness is not related to the flu (influenza), which is a respiratory illness caused by influenza virus. Centers for Disease Control and Prevention has more information (and a fact sheet [300 kb PDF]). California Department of Public Health also has additional information about norovirus.

Waterborne Diseases

To protect yourself from disease, treat any river or lake water before drinking. The most reliable way to disinfect water is to boil it for three minutes. Other options include:

  • Use a filter rated to remove bacteria and protozoa (1 micron or smaller filter). After filtering, treat the water with 2 drops of chlorine bleach per quart (liter) of water for 30 minutes in order to kill viruses (which are not removed by filters). Viruses aren't common in Yosemite's waters, but we recommend that you treat water to kill them.
  • Use a purifier rated to remove viruses. Check your filter or purifier carefully to understand what it is designed to remove.

To prevent the spread of Giardia and other water-borne disease organisms, use restroom facilities where available, and wash or sanitize your hands often. In natural areas where facilities are not available, bury human waste six inches deep and at least 100 feet away from any water source or trail. Also, do your washing and camping at least 100 feet away from any water source or trail.

Air Quality

Burning of fossil fuels and some agricultural emissions west of Yosemite, along with smoke from fires in and near Yosemite, degrade Yosemite's air quality, mainly during summer. Smoke and poor air quality may irritate your eyes and respiratory system, and may worsen chronic heart and lung conditions. (Check current air quality conditions.) The Centers for Disease Control and Prevention has additional information about air pollution and respiratory health and health threats from wildfire smoke.

Carbon Monoxide Poisoning

Carbon monoxide (CO) is an odorless, colorless gas that can cause sudden illness and death. Campers using gas-powered generators, stoves, lanterns, etc., run the risk of CO poisoning if they use these devices in areas without adequate ventilation.

Last updated: February 19, 2024

Park footer

Contact Info



Contact Us