Skip to content


Glossary Navigation

Active immunity

When you are protected against a disease because your immune system is making antibodies to fight that disease. People get active immunity one of two ways, either by getting the disease or through vaccination. Active immunity is sometimes permanent, meaning a person is protected from the disease for the rest of their life.


An illness or injury that happens suddenly and noticeably.


Adjuvants are “booster” ingredients – they help your body produce a stronger immune response which means greater protection against a disease. The advantage of adjuvants is that they help produce more antibodies and longer-lasting immunity so a smaller amount of the virus or bacteria needs to be used in the vaccine.

Adverse events

An “adverse event” is any health problem that happens after a shot or other vaccine. An adverse event might be truly caused by a vaccine, or it might be pure coincidence.

Advisory Committee on Immunization Practices (ACIP)

A group of medical and public health experts who develop recommendations on the use of vaccines in the United States. The members of this committee are not government employees.


When your body has an exaggerated response to a substance (e.g., food or drug). Also known as hypersensitivity.


An immediate and severe allergic reaction to a substance (e.g., food or drugs). Symptoms of anaphylaxis include breathing difficulties, loss of consciousness and a drop in blood pressure. This condition can be fatal and requires immediate medical attention.


A medicine that fights bacteria.


An antibody is part of the immune system that recognizes foreign substances like bacteria and viruses (antigens) and fights them. Once an antibody encounters an antigen it will remember that antigen and protect against future attacks.


Foreign substance (e.g., bacterium or virus) that is capable of causing an immune response in the body. The presence of antigens in the body triggers the immune system to act, usually producing antibodies and other immune cells.


A solution of antibodies against a toxin. Antitoxins give passive immunity and can help treat diseases caused by toxins.


Literally “against-virus” — any medicine capable of destroying,  weakening, or treating a virus.


Joint pain.


A medical condition with inflammation in the joints which results in pain and difficulty moving. It may be temporary or long-lasting.


The terms association and relationship are often used interchangeably. When two things happen close together in time or when one thing causes another thing. See causal and temporal association.


An ongoing medical condition where the bronchial tubes (in the lungs) become easily irritated. This causes wheezing, coughing, difficulty breathing and production of thick mucus. Why someone gets asthma is not yet known but environmental triggers, drugs, food allergies, exercise, infection and stress have all been implicated.

Asymptomatic infection

The presence of an infection without symptoms. Also known as subclinical infection. People with asymptomatic infections can sometimes spread an illness. 

Attenuated vaccine

A vaccine containing a virus that is so weakened it cannot make you sick but your body can still recognize the virus and produce antibodies to fight it. Attenuated vaccines currently licensed in the United States include measles, mumps, rubella, varicella, rotavirus, yellow fever, and some formulations of influenza and typhoid vaccines.


Autism is a neurodevelopmental difference that affects how autistic people experience the world around them. Autistic people are an important part of the world. Autism is a normal neurological variation.

B cells

Small white blood cells that help the body defend itself against infection. These cells are produced in bone marrow and develop into plasma cells which produce antibodies. Also known as B lymphocytes


Microscopic one-celled organisms found everywhere including on and in our bodies. While some bacteria are helpful, some cause illness. Examples of dangerous but vaccine preventable bacterial diseases include diphtheria, pertussis, tetanus, Haemophilus influenzae, and pneumococcal disease.

Biological plausibility

A relationship between two things or a situation in which something could occur based on how our bodies work.

Booster shots

Additional doses of a vaccine needed periodically to remind the immune system how to fight a disease. For example, the tetanus and diphtheria (Td) vaccine which is recommended for adults every ten years.

Breakthrough infection

When a person gets sick with the disease despite having been vaccinated against it. Breakthrough infections are uncommon but no vaccine is 100% effective.

Causal association

A relationship between two things where one thing makes a change in the other one happen. For instance, smoking has a causal association with lung cancer.


See Varicella.

Chronic health condition

An illness or injury that lasts for a long period of time (e.g., cancer, asthma).

Combination vaccine

Combination vaccines take two or more vaccines that could be given individually and put them into one shot or oral dose.


A disease that can be passed from one person or animal to another.

Community immunity

Community immunity happens when enough people in a community are immune to a disease that they “shield” those who don’t have immunity from getting the disease. This immunity can come from the disease itself but that means that a lot of people got sick.  It’s much safer for the immunity to come from vaccination. Also known as herd immunity.

Conjugate vaccine

The addition of a compound (e.g., a sugar or a protein) to an antigen that increases a vaccine’s effectiveness.


A condition which makes a particular treatment or procedure inadvisable. For example, someone undergoing chemotherapy may be advised to wait on some vaccines, particularly live viral vaccines.


See Seizure

Demyelinating disorders

A medical condition where the myelin sheath, which surrounds nerves, is damaged. These nerves are responsible for the transmission of impulses to the brain. Damage to the myelin sheath results in muscle weakness, poor coordination and possible paralysis. Examples of demyelinating disorders include Multiple Sclerosis (MS), optic neuritis, transverse neuritis and Guillain-Barre Syndrome (GBS).


A highly contagious and sometimes fatal bacterial disease that causes mucous membrane inflammation, making breathing and swallowing difficult. Diphtheria can also cause swelling of the heart and nerves.


Sickness, illness, or loss of health.


A measure used to describe how good a vaccine is at preventing disease. This rate measures how a vaccine performs in the real world, outside of clinical trials.

Efficacy rate

A measure used to describe how good a vaccine is at preventing disease. This rate measures how a vaccine performs under ideal situations in clinical trials.


Reduction of the spread of disease to zero in a defined region through prevention efforts like vaccination. Cases can still be spread from people traveling into the region from other places where the disease has not been eliminated.

Emergency Use Authorization

When FDA temporarily allows a vaccine to be given to the general public before it has gone through the full licensure process. This can only be done during a public health emergency where the vaccine can reduce cases and deaths. The vaccine will still need to go through the full licensure process.


Swelling of the brain caused by a virus or bacteria. Encephalitis can result in permanent brain damage or death.


A general term describing damage or disease that affects the brain. Examples include encephalitis, meningitis, seizures, and head trauma.


When a disease is normal and expected among particular people or in a certain area.


When a disease spreads beyond what is normal or expected, infecting a lot of people in a specific population or area.


Permanently wiping a disease from the face of the earth. To date, smallpox is the only human disease that has been eradicated although there is an ongoing worldwide effort to eradicate polio.


The cause of.


When you come in contact with bacteria, viruses, or other substances that can make you sick.


Relating to fever; feverish.

Guillain-Barre Syndrome (GBS)

A rare neurological disease characterized by loss of reflexes and temporary paralysis. Symptoms include weakness, numbness, tingling and increased sensitivity that can spread over the body. GBS can cause muscle paralysis and breathing difficulties. Most people recover but it can have permanent effects or cause death. 

Haemophilus influenzae type b (Hib)

A bacterium that may result in severe respiratory infections, including pneumonia and other diseases such as meningitis.

Hepatitis A

A highly contagious viral disease people can catch eating or drinking contaminated food or water. While Hep A will go away on its own after a month or two for most people, it can be more serious or fatal for adults.

Hepatitis B

Hepatitis B is a serious liver infection passed person-to-person through blood and other bodily fluids. Pregnant women can pass Hepatitis B to their babies during childbirth, which is why newborns are given a Hep B vaccine at birth.

Herd immunity

See Community immunity

Herpes Zoster (Shingles)

Shingles is a viral infection caused by the same virus that causes chickenpox (varicella virus). Anyone who has had chickenpox can get shingles, but it is most common over age 50. A person can get shingles more than once.   

Shingles causes a painful rash with localized blisters. While the condition is not contagious, it is possible to pass chickenpox to another person if they are in contact with fluid from the blisters, which is why people with shingles should cover their rash until the blisters crust over. 


Red, raised marks on the skin that appear suddenly. It is usually itchy or painful. Hives can be caused by an allergy, stress, infection or even heat and cold. Also known as urticaria.


When the body has an exaggerated response to a substance (e.g., food or drug). Also known as an allergy.

Immune globulin

A protein found in the blood that fights infection. Also known as gamma globulin.

Immune system

The complex system in the body responsible for fighting disease. Its primary function is to identify foreign substances in the body (bacteria, viruses, fungi or parasites) and develop a defense against them. This defense is known as the immune response.


Protection against a disease. Immunity may be indicated by the presence of antibodies in the blood, determined with a laboratory test. There are two types of immunity, active and passive.


When someone receives a vaccine and mounts an immune response to protect against a disease.


When the immune system is unable to protect the body from disease. This condition can be caused by disease (like HIV infection or cancer) or by certain drugs (like those used in chemotherapy). Individuals whose immune systems are compromised should not receive live, attenuated vaccines.

Inactivated vaccine

A vaccine made from viruses and bacteria that have been killed through physical or chemical processes. These killed organisms cannot cause disease.


The number of new disease cases in a population over a certain period of time.

Incubation period

The time between when you come in contact with bacteria or viruses to when you start feeling sick.


Capable of causing disease.

Infectious agents

Organisms capable of causing disease (e.g., bacteria or viruses).


Redness, swelling, heat and pain in tissues (e.g., organs or muscles) resulting from injury or illness. Also known as swelling.


Influenza (more commonly known as the flu) is a highly contagious viral illness that is spread person-to-person through sneezes and coughs. It usually is a seasonal illness (October – May in the U.S.) and can be serious and life-threatening. One of the reasons the flu is so contagious is that people can pass it on before they even know they are sick.


A type of bowel blockage that happens when one portion of the bowel slides into the next, much like the pieces of a telescope; it is treated in a hospital and may require surgery.

Investigational vaccine

A vaccine that has been approved by the Food and Drug Administration (FDA) for use in clinical testing in humans. However, investigational vaccines are still in the testing and evaluation phase and are not licensed for use in the general public.


Yellowing of the skin and eyes. This condition is often a sign of hepatitis infection.

Live vaccine

A vaccine using a virus or bacteria weakened so much that it cannot make you sick but can work with the body to create a good immune response. Live vaccines currently licensed in the United States include measles, mumps, rubella, varicella, rotavirus, yellow fever, and some formulations of influenza, and typhoid vaccines.  Also known as an attenuated vaccine.


Small white blood cells that help the body defend itself against infection by remembering a previous battle with that infection.


A large cell that helps the body defend itself against disease by surrounding and destroying viruses or bacteria.


Measles is the most contagious viral illness. It is spread from person to person through coughing or sneezing.  Common symptoms include a cough, runny nose, inflamed eyes, sore throat, fever, and a red blotchy skin rash, and it can last for 5-7 days.

Memory cells

A group of cells that help the body defend itself against disease by remembering prior exposure to specific viruses or bacteria. These cells help your body respond quickly.


Inflammation of the covering of the brain and spinal cord (the meninges) that can result in permanent brain damage and death.


Tiny organisms (including viruses and bacteria) that can only be seen with a microscope.

Mucosal membranes

The soft, wet tissue that lines body openings specifically the mouth, nose, rectum, vagina, and other bodily openings.


Mumps is another contagious viral illness that affects the glands around the ears, causing puffy cheeks and a swollen, painful jaw. It can also affect the testicles, the pancreas, and several other organs. It is spread from person to person with close contact, such as those sharing a home, practicing sports with others, or sharing cups or water bottles.


Inflammation of the nerves.


Weakness, numbness, pain, or all three caused by damage to the nerves outside of the brain and spinal cord.


Inflammation of one or both testicles, often a complication of mumps. Most patients recover but in rare cases sterility occurs.


When cases of a disease spreads beyond what is normal, infecting a lot of people in a defined population or area.


A worldwide epidemic (e.g., COVID-19)

Passive immunity

Protection against disease when you are given another person or animal’s antibodies, or manufactured antibodies. While passive immunity takes effect immediately it is not as strong or long lasting as active immunity. For example, a mother’s antibodies are passed to the infant prior to birth. These antibodies temporarily protect the baby for the first 4-6 months of life.


Organisms (e.g., bacteria, viruses, parasites and fungi) that cause disease.

Pertussis (whooping cough)

Pertussis, or Whooping Cough, is a bacterial infection that causes violent, uncontrollable coughing and difficulty breathing. It is sometimes called the 100-day cough for its long-lasting effects. Pertussis can affect anyone but is extremely serious and sometimes deadly for babies under one year of age.


[“pe TEEK ee eye”] — Tiny reddish or purplish spots on the skin or mucous membrane. These spots do not turn white when pressed. Petechiae has many causes such as trauma, injury, and viral or bacterial infections.


A substance that cannot prevent or treat disease. In vaccine testing, the placebo may contain everything that is normally in the vaccine except the active ingredient. A placebo is used in clinical trials to test the effectiveness of vaccines. People in one group get the vaccine, while the others receive the placebo.


Pneumonia is an infection of the lungs that can cause them to fill with fluid. Bacteria, viruses, or fungus can cause it. Bacterial and viral pneumonia can be spread from person to person.

Poliomyelitis (polio)

Poliomyelitis (polio) is a viral infection that is spread person-to-person through the fecal-oral route. Children who have polio shed the virus in their excrement (poop). It is most commonly spread through unclean hands, contaminated food, and untreated water. Some people with people will have no symptoms or mild symptoms. For others, polio infects the spinal cord and can cause permanent paralysis or even death.

Polysaccharide vaccine

Vaccines that are composed of long chains of sugar molecules that resemble the surface of certain types of bacteria. Polysaccharide vaccines are available for pneumococcal disease.


A measure of strength.


A condition in a person that might increase the risk for a serious adverse reaction or decrease the effectiveness of a vaccine. A precaution is not a contraindication. It may not prevent a person from getting a vaccine.


The total number of cases of a disease within a population over a given time period.


The time before the onset of an attack or a disease.


The isolation of a person or animal who has been exposed to a disease in order to prevent further spread of the disease.


How much or how often a vaccine produces a known, “expected” reaction, especially such as fever or sore arm at the injection site. The reactogenicity of a vaccine does not mean it is safe or unsafe.


Recombinant vaccines use specific pieces of different viruses to help the immune system recognize a disease without causing illness.


The likelihood that an individual will experience a certain event.


A group of highly contagious viruses that can cause severe diarrhea, mostly in children. Rotavirus is particularly dangerous in infants.

Rubella (German Measles)

A viral infection with a red rash, enlarged neck lymph nodes, mild fever, and headache. It is spread person to person by coughing or sneezing. Rubella is highly contagious but was declared eliminated in the U.S. Parents should continue to vaccinate their children to prevent rubella from coming back. 


See Measles.


A seizure is a sudden, uncontrolled electrical disturbance in the brain that causes a jerking or staring spell. Seizures are also known as convulsions. A common cause of seizures in young children and infants is fever. If a child has a febrile seizure after being vaccinated, keep mind that the fever, not the vaccine, causes the seizure. While these can be frightening for parents, these types of seizures are usually not harmful. While children will outgrow febrile seizures, Epilepsy is a chronic seizure disorder that can affect people for life.


Development of detectable antibodies in the blood of a person who previously did not have detectable antibodies.


Measurement of antibodies, and other immunological properties, in the blood.


Viral shedding happens when a virus replicates inside your body and is released into the environment. You may not have symptoms but can still be contagious. Live virus vaccines can also shed the vaccine virus but vaccine shedding is unlikely to make a person sick.


See herpes zoster.

Side effect

A response that happens as a result of a vaccine, such as redness or swelling at the injection site. Most vaccine side effects are expected and will go away on their own in a day or two.


An acute, highly contagious, often fatal viral disease and characterized by high fever and aches with subsequent widespread eruption of pimples that blister, produce pus, and form pockmarks. Smallpox has killed millions of people but has not made anyone in the world sick since 1977.


A specific version of an organism. Many viruses and bacteria, including influenza, polio, and pneumococcal bacteria, have multiple strains. Scientists can make vaccines that fight multiple strains of the same organism.

Subclinical infection

When a person is infected with germs but has no symptoms. It is important to know that someone with subclinical infection may still pass the germs to others. Also known as asymptomatic infection.

Sudden Infant Death Syndrome (SIDS)

The sudden and unexpected death of a healthy infant under 1 year of age. A diagnosis of SIDS is made when an autopsy cannot determine another cause of death. The cause of SIDS is unknown. SIDS cases have dropped since safe sleeping campaigns (ABC: Alone, on their Back, in a Crib) began. Also known as “crib” or “cot” death.


When you are at risk of getting a disease because you are not protected against it.

Temporal association

Two or more events that occur around the same time but might be unrelated, chance occurrences. For example, a child receives an MMR vaccine around 1 year old – the same time most children start walking.  But the MMR vaccine did not cause the child to walk.


A disease caused by a bacterium found in the environment (like soil and manure). The infection causes muscle spasms and stiffness, which can last for weeks. Breathing can be affected by these symptoms. 10-20% of people with tetanus will die, even with treatment. Neonatal tetanus kills 100% of the newborns who get it.


A once commonly used preservative in multi-dose vaccine vials, thimerosal breaks down into ethylmercury and is excreted from the body. Thimerosal has been used in vaccines, other drugs, and even contact lens solutions since the 1930s. Today, there are no routine childhood vaccines that use Thimerosal. A small percentage of flu vaccines given to children might contain Thimerosal.


The detection of antibodies in blood through a laboratory test.

Typhoid fever

Typhoid fever is a life-threatening illness caused by the bacterium Salmonella typhi. People with typhoid fever may carry the bacteria in their bloodstream and intestinal tract, some for a long time.


Red, raised marks on the skin that appear suddenly. It is often itchy or painful. Hives can be caused by an allergy, stress, infection or even heat and cold. Also known as hives.


When someone gives a vaccine to a person or animal.


A substance that helps your body develop immunity against one or several diseases without making you sick from the disease(s).

Vaccine Adverse Event Reporting System (VAERS)

VAERS is a safety monitoring system that accepts reports of adverse events that happen after vaccination. Anyone can submit a report to VAERS, but submissions do not mean that a vaccine caused the event. VAERS is one of several monitoring systems

Vaccine Safety Datalink Project (VSD)

A system that monitors vaccine adverse events. This project contains data on more than 12 million people. Researchers work to analyze this data to determine if a vaccine is causing adverse events. 

Varicella (Chickenpox)

Chickenpox is a very contagious disease caused by the varicella-zoster virus. Chickenpox causes an itchy rash that scabs over and usually lasts about a week. Scratching the scabs can cause dangerous skin infections and permanent scarring. Chickenpox can also cause fever, tiredness, loss of appetite, headache, and even death. A person who has had chickenpox can develop shingles later in life.


How much an organism, such as a virus or bacterium, can cause severe disease.


A tiny organism that multiplies within cells and causes diseases such as chickenpox, measles, mumps, rubella, and hepatitis. Antibiotics have no effect on viruses.

Waning immunity

The loss of protection against a disease over time.

Whooping cough

See Pertussis.

Back To Top