Influenza is caused by viruses that infect the cells that line the nose, throat and lungs.

Flu virus particles spread through breath, saliva, mucus or stool. Bird flu in humans can happen when you inhale virus particles. You also can catch the virus if you touch an object with flu particles on it, and then touch your eyes, nose or mouth.

People most often catch bird flu from close, long-term contact with live, domesticated poultry typically on farms or in backyard coops. Rarely, people are exposed to bird flu by contact with wild birds or another type of animal.

But birds you may see in a park or yard, such as crows or sparrows, aren't a high risk. They don't usually carry bird flu viruses that infect people or farm animals.

It may be possible to be exposed to bird flu through undercooked foods, such as eggs or poultry. In places where bird flu has spread to dairy cows, it may be possible to get bird flu through raw dairy products. But dairy products that have been heated to kill germs, called pasteurization, are not a risk for bird flu.

People with bird flu may have worsening medical issues or new health problems. Some may be life-threatening.

Complications include:

• Worsening of chronic lung conditions, such as asthma or cystic fibrosis.
• Ear and sinus infection.
• Failure of the respiratory system, called acute respiratory distress syndrome.
• Kidney problems.
• Heart problems.
• Bleeding in the lungs, collapsed lung or bacterial pneumonia.
• Sepsis.

To diagnose bird flu, a healthcare professional takes a sample of fluid from your nose and throat to test for evidence of bird flu infection. They also may take a sample of fluid from your eye if you have pink eye.

If you could have been exposed to bird flu through work, travel or your hobbies and you have symptoms of illness, see your healthcare professional right away.

Bird flu, also called avian influenza, is caused by influenza type A virus infections in bird species. Depending on the strain, bird flu may cause the bird to have no symptoms, mild illness, serious illness or lead to the death of the bird.

Bird flu rarely infects humans. But health officials worry because influenza A viruses that infect birds can change, called mutate, to infect humans and spread from person to person more often. Because a new strain of bird flu would be a new virus to humans, a mutated strain like that could spread quickly around the world.

People most often catch a bird flu virus from close, long-term contact with live, domesticated poultry typically on farms or in backyard coops. People also may catch bird flu through contact with wild birds or another type of animal. Bird flu rarely has spread from person to person.

In humans, flu is a viral infection of the nose, throat and lungs, which are part of the respiratory system. Symptoms of bird flu in humans are similar to flu symptoms and may be mild to serious.

To prevent bird flu, follow all recommended actions to protect yourself if you work with animals as a job.

If you are traveling to a place where bird flu is spreading, avoid poultry farms and bird markets if possible. Cook food fully and wash hands with soap and water after handling food and animals.

And make sure to get your seasonal flu vaccine every year. The U.S. Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. It doesn't prevent bird flu, but the seasonal flu vaccine can help you avoid having two flu viruses at the same time.

If a bird flu virus leads to a human pandemic, public health agencies have plans for vaccine development and administration.

People can take actions to further lower the risk of getting bird flu in many ways.

• Avoid contact with animals who are sick or may be sick. Wild or domestic, keep birds at a distance to avoid any germs they may carry.
• Wear eye, nose and mouth protective gear when needed. Flu viruses get in the body through the mouth, nose or eyes. Wear eye protection, a face mask and gloves to help keep the virus out if you're in an area where it might be present.
• Wash your hands with soap and water. This is especially important after touching animals or surfaces that may be dirty with animal mucus, saliva or stool.

Getting bird flu from food is very rare. But it's a good idea to follow safe food handling recommendations.

• Avoid spreading germs in the kitchen. Use hot, soapy water to wash all surfaces that have come into contact with raw poultry, meat, seafood or eggs.
• Cook food fully. Cook chicken until it reaches an internal minimum temperature of 165 F (74 C). Cook eggs until the white and yolk are firm. Egg dishes, such as quiche, should reach 160 F (71 C). Cook beef to 145 F (63 C) and let it rest for 3 minutes. Cook ground beef to 160 F (71 C).
• Avoid raw dairy products. Dairy milk that is heated to kill germs is called pasteurized. In the U.S., dairy milk and foods made with it say on the Nutrition Facts label if the milk is pasteurized. Raw milk isn't pasteurized, so it is more likely to make you sick.

The risk of a human catching a bird flu is low.

Contact with sick poultry or their environment is the most common bird flu risk for people. Infected birds can spread the virus through their breath, saliva, mucus or stool.

Rarely, people have caught bird flu after contact with wild birds or other animals. And sometimes humans have passed a bird flu to other humans.

Bird flu symptoms can be mild to serious in a person. Symptoms typically show up within seven days of contact with the virus but can take as long as two weeks. A person can get infected from direct contact with an infected animal, or the bedding or stool of the animal.

Flu viruses have similar symptoms. So you need to be tested to check if you have a bird flu infection.

Common bird flu symptoms include:

• Fever.
• Problems breathing.
• Pink eye, also called conjunctivitis.
• Upset stomach and vomiting.
• Loose stool, called diarrhea.

Bird flu may cause breathing problems more often than other kinds of flu. And during bird flu pandemics, the risk that a person with flu will need a machine to help them breathe is higher.

When to see a dotor

If you have been exposed to bird flu and have any symptoms of illness, see a healthcare professional right away.

If your work, travel or hobbies may have exposed you to bird flu, consider your symptoms. If you have symptoms of bird flu and may have been exposed, contact your healthcare professional.

Medicines that stop the flu virus from spreading in the body, called antivirals, work to treat bird flu. These medicines work best when started as soon as you have symptoms. For this reason, your healthcare professional may prescribe a medicine for you before your flu lab test results comes back.

These medicines can include oseltamivir (Tamiflu), zanamivir (Relenza) or peramivir (Rapivab).

Oseltamivir is taken by mouth. Zanamivir is inhaled using a device similar to an asthma inhaler. Peramivir is given through a needle in a vein.

If you have bird flu, stay in an area away from other people, including those you live with if possible. That will help prevent spreading the illness.

It's important to avoid people if you're waiting for lab results. If you do have bird flu, healthcare professionals may suggest testing for people who had close contact with you when you had symptoms. They may give antiviral medicine to people who were exposed to bird flu and are at high risk of serious illness.