Herd immunity is the name for a point in time when it's hard for a disease to spread through a group of people. The idea of herd immunity works for some diseases, such as measles. But it's a harder concept to apply to illnesses like coronavirus disease 2019 (COVID-19).

Read on to learn how herd immunity makes sense for some diseases but not others.

Herd immunity is important because it defines when a whole community is protected. That includes people who haven't caught the disease, people who had the illness and recovered, and people who got a vaccine. It also includes people who can't get a vaccine.

Data on herd immunity helps guide vaccine goals set by public health agencies.

Herd immunity can't be reached for every disease, but measles is one example of the idea. Measles is a disease caused by a virus that spreads quickly among people who've never had the disease or the measles vaccine.

As people recover or get a measles vaccine, the virus has fewer new people to infect.

The virus that causes measles doesn't change much, called mutate, over time. That means once you get the vaccine for measles or recover from the illness, you are not likely to get it again.

Based on those facts, health officials estimate that herd immunity for measles is at least 94%.

That means 94 people out of 100 in a population need to be immune to stop the spread of the measles virus. That includes measles recovery or people who got both measles vaccine shots.

So keeping at least 95% of people vaccinated against the measles virus is a public health goal. At that level, people who can't get the vaccine, such as children younger than 12 months, are protected.

Herd immunity for illnesses such as measles and polio happens when you and the people around you get vaccinated.

Before the vaccine for measles, millions of people got the disease. In the U.S., hundreds died of measles each year and thousands needed care in the hospital.

After people could get the measles vaccine, the measles virus stopped spreading in the U.S. because so many people got the shots.

Each year, there are still outbreaks of measles. These are mostly among people who haven't had a vaccine, are undervaccinated or who have a breakthrough illness. But the number of people in the U.S. who get measles is in the hundreds, not millions.

But herd immunity can be lost. If people can't get a vaccine, or choose not to, the protection among a population goes down.

During the COVID-19 pandemic, children worldwide missed getting a measles vaccine. Those missed doses led to an increase in measles cases and deaths in 2022 compared with 2021.

And the idea of herd immunity doesn't work for every disease.

• Herd immunity may not be possible when viruses change a lot in a short time, as with the virus that causes COVID-19.
• Reaching herd immunity is harder if a disease can be spread by people who catch the virus but don't have symptoms.
• Herd immunity is much harder to achieve if the protection from having and recovering from the illness or getting a vaccine doesn't last a long time.

Spread of the viruses that cause COVID-19, flu and RSV are examples of when herd immunity may not be a realistic goal. With this type of illness, the goal is to control and limit the spread of the virus.

Getting vaccinations as they are updated and on schedule helps lower the risk of getting sick. Testing when you have symptoms to know when you need to avoid other people can help prevent spreading a virus.

One key action you can take is to wash your hands.

Wash your hands well and often with soap and water for at least 20 seconds. If you can't use soap and water, use an alcohol-based hand sanitizer with at least 60% alcohol. Make sure people around you, especially children, know the importance of hand-washing and how to do it correctly.

If you can, try to avoid being in crowded, indoor spaces with poor airflow when respiratory viruses are spreading.

You also can take other actions to prevent the spread of germs:

• Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.
• Avoid touching your face. Keeping your hands away from your eyes, nose and mouth helps keep germs from entering the body there.
• Clean surfaces. Regularly clean often-touched surfaces to prevent the spread of viruses from a surface to your face.
• Help others from afar. If you can, avoid close contact with anyone who is sick or has symptoms.

When respiratory illness germs are spreading in your area, wearing a mask can give you another layer of protection. These types of germs spread when people talk, sneeze, cough or sing, for example.

People who are at high risk of serious illness, or who are regularly around people at high risk, may choose to wear a mask. If you came in contact with a germ, are sick or are getting over sickness, wearing a mask can help protect the people around you.

In the U.S., the Centers for Disease Control and Prevention suggests wearing the most protective mask possible that you'll wear regularly, fits well and is comfortable.

If you have a chronic medical condition and may have a higher risk of serious illness, check with your healthcare professional about other ways to protect yourself.