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, or IR3535 typically provide reasonably long-lasting protection against mosquitoes, but the Centers for Disease Control and Prevention recommends repellents containing DEET or permethrin to repel ticks. (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


Like all national parks, we are closely monitoring the news of the spread of the Coronavirus (COVID-19). Comprehensive, accurate, and up-to-date information about the virus can be found on the CDC's website. The park taking steps to prevent illness and mitigate any impacts.

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.

September 2022 Update

Recent testing yielded positive results for small amounts of toxic algae in El Portal, Yosemite Valley, Wawona, and the Grand Canyon of the Tuolumne. Toxins are concentrated within the algal mats themselves and released episodically into the water when the algae dies or is disturbed.

What does this mean for me?

  • For your safety, do not enter or drink water in areas of dense algal mats.

  • Filtering and/or boiling the water is not effective against this type of algae.

  • Prevent pets from drinking the water and eating or touching algae in the water and dried on the shore. In particular, prevent dogs from eating dried algal mats on shore.

  • Please report any large algal blooms and/or algae that is particularly bright, bubbly, strange-looking, or appears like a haze in the water.

  • Do not disturb algal mats in any way. Wading or swimming where algae is present can cause toxins to be released into the water.

  • If you suspect a site has toxic algae, do not enter the water and do not drink water from the area. While some sites are signed based on testing results, it’s likely that algae exists in other parts of Yosemite. Don’t rely on signage alone.

Can I still swim in the river? Can I still filter or treat the water for drinking?
Water that is clear with no visible algae in the area presents a low risk. Even in areas with no visible algae, watch for isolated clumps of algae floating by.

If you think algae may be in the water:

  • Do not enter the water.

  • Do not drink the water, even if treated.

  • Do not let pets into the water, allow them to drink the water, or eat algae on the shore.

What are the signs and symptoms of exposure to toxins from algae?

According to the California Water Quality Monitoring Council, the following signs and symptoms may occur within 48 hours of exposure to a waterbody with a suspected or confirmed algal bloom:

  • sore throat or congestion;

  • coughing, wheezing, or difficulty breathing;

  • red, or itchy skin, or a rash;

  • skin blisters or hives;

  • earache or irritated eyes;

  • diarrhea or vomiting;

  • agitation;

  • headache; and/or,

  • abdominal pain.

If people show symptoms of cyanotoxin and/or cyanobacteria exposure after contact with water, or with scums or mats of algae, they should receive immediate medical attention. Additional resources are available from the Centers for Disease Control and Prevention (CDC), and by contacting the California Poison Control Center (1-800-222-1222). See the HAB-related Illness Tracking webpage for information on previously reported human illnesses related to HABs in California.

Is the park monitoring for harmful algal blooms?

The park continues monitoring for toxic algae and testing for toxins throughout the park. You can learn more about harmful algal blooms at


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: October 24, 2022

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