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People are protected from illness directly and indirectly. Direct protection is called active immunity. Indirect protection is called passive immunity. Another type of immunity that you might hear people talk about is community (or herd) immunity. Find out more about each of these.

Active immunity

Active immunity is protection provided by our own immune system. We become immune in two ways:

1. Infection — Our immune system responds to protect us from pathogens in the environment. Sometimes we know we are infected because we experience symptoms of illness. Other times, we do not experience symptoms. This means we don’t know our immune system was busy protecting us.
2. Vaccination — When we are vaccinated, our immune system responds the same way it would if we were infected by a pathogen from the environment. We are aware that it is a vaccine and not a pathogen. But, our immune system is not. Vaccination offers a few benefits over infection:
   • When: We can control when immunity develops. The vaccine schedule is set up so that people who follow it can build immunity before they are at higher risk of exposure. For example, the hepatitis B vaccine is the first one given within hours of birth. Hepatitis B virus can cause infection through exposure to amounts of infected blood that are too small to see with the eye, so babies are vaccinated soon after they are born. This helps if mom is infected, and it helps if a family member, caregiver, or other person with whom the baby comes into contact is infected. Each year before the newborn dose was given, about half of children infected were exposed to someone other than their mother who was infected. Since children often don’t have symptoms when they are infected, they can grow up with a virus they don’t know they have until they develop severe liver disease or liver cancer. This not only often shortens their own lifespan, but it also means they may spread a virus they don’t know they have.
   • What: We know which pathogens are likely to cause illness. So, vaccines can be designed to generate protection against the most dangerous ones. For example, the HPV vaccine protects against nine of the more than 100 types of HPV that most often cause cancer and genital warts in males and females.
   • How: Scientists can determine which parts of a pathogen cause illness. This means that the vaccine can target the parts that generate protective immunity. For example, two proteins of influenza virus are the most troublesome. They are named hemagglutinin and neuraminidase. The annual influenza vaccine is made to match these two proteins on the surface of influenza strains that are causing illness in males and females.
   • Why: Scientists complete studies to figure out the lowest quantity of the pathogen that will create immunity without causing symptoms. In this way, people can become immune without getting sick. Vaccines reproduce only a few times or not at all. This means that illness and complications from natural infections are not typically a concern. For example, the chickenpox vaccine virus reproduces a few times. But, it is a weakened virus. This means a child may get at most only a few blisters. In contrast, illness caused by natural chickenpox virus will last about a week and reproduce hundreds or thousands of times. This will result in symptoms such as hundreds of blisters, fever, tiredness, headache, vomiting and diarrhea. Both the vaccine virus and the natural virus can live silently in the nervous system. But, since the vaccine version is weakened, it is much less likely to reactivate and cause shingles in adults with weakened immunity.

Passive immunity

Passive immunity is “borrowed immunity.” This means that it relies on antibodies generated by someone else’s immune system. Two examples are:

1. Maternal antibodies — A baby receives antibodies through the placenta during pregnancy. These antibodies will live for a short time and circulate in the baby’s bloodstream, giving their own immune system time to make antibodies. Babies also get maternal antibodies from breast milk. The breast milk made during the first few days after birth is called colostrum. It has greater amounts of antibodies than later breast milk. Antibodies in breast milk coat the surface of the throat and intestines, but they don’t enter the baby’s bloodstream.   
2. Immunoglobulin treatments — Sometimes, a patient will get antibodies as a form of treatment. For example, people may be given antivenom after snake bites. Antivenom is a mixture of antibodies isolated from someone else who was bitten by the same type of snake. Babies born during RSV season can now get an antibody treatment shortly after birth to protect them during their first RSV season. By their second RSV season, most babies will have been exposed to the virus and their immune system will have generated antibodies to protect them.

The biggest drawback of passive immunity is that it is short-lived. Ultimately, we need immunity generated by our own immune systems to be protected for any length of time.

Learn more about a baby’s immune system at birth on the VEC website.

Community, or herd, immunity

Community immunity is another type of immunity you may have heard about. It is also called herd immunity. This type of protection is different from active or passive immunity because it does not involve specific parts of the immune system, such as antibodies. Instead, community immunity relies on being surrounded by people who are immune. People who are susceptible to a disease are less likely to be infected because the pathogen has fewer chances to spread when most people are immune. 

For example, think of the community as a building with lots of connected rooms. Imagine that each room represents a person. If the rooms have open doors, an invader will have an easier time reaching any particular room. But, if most or all of the doors are closed, the invader will get into fewer rooms and be less likely to find a particular room where the door is open.

Reviewed by Paul A. Offit, MD, on Nov. 20, 2025