Medical Terminology Definitions

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NOTE: All information is intended for your general knowledge only and is not a substitute for medical advice or treatment for specific medical conditions.

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Definitions for Medical Terminology

NOTE: All information is intended for your general knowledge only and is not a substitute for medical advice or treatment for specific medical conditions.

Acellular vaccine

A vaccine containing partial cellular material as opposed to complete cells.

Active immunity

The production of antibodies against a specific disease by the immune system. Active immunity can be acquired in two ways, either by contracting the disease or through vaccination. Active immunity is usually permanent, meaning an individual is protected from the disease for the duration of their lives.


A short-term, intense health effect.


A substance (e.g. aluminum salt) that is added during production to increase the body's immune response to a vaccine.

Adverse events

Undesirable experiences occurring after immunization that may or may not be related to the vaccine.

Advisory Committee on Immunization Practices (ACIP)

A panel of 10 experts who make recommendations on the use of vaccines in the United States. The panel is advised on current issues by representatives from the Centers for Disease Control and Prevention, Food and Drug Administration, National Institutes of Health, American Academy of Pediatrics, American Academy of Family Physicians, American Medical Association and others. The recommendations of the ACIP guide immunization practice at the federal, state and local level.


A condition in which the 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 protein found in the blood that is produced in response to foreign substances (e.g. bacteria or viruses) invading the body. Antibodies protect the body from disease by binding to these organisms and destroying them.


Foreign substances (e.g. bacteria or viruses) in the body that are capable of causing disease. The presence of antigens in the body triggers an immune response, usually the production of antibodies.


Antibodies capable of destroying microorganisms including viruses and bacteria.


Joint pain.


A medical condition characterized by inflammation of the joints which results in pain and difficulty moving.


The degree to which the occurrence of two variables or events is linked. Association describes a situation where the likelihood of one event occurring depends on the presence of another event or variable. However, an association between two variables does not necessarily imply a cause and effect relationship. The term association and relationship are often used interchangeably. See causal and temporal association.

Asymptomatic infection

The presence of an infection without symptoms. Also known as inapparent or subclinical infection.

Attenuated vaccine

A vaccine in which live virus is weakened through chemical or physical processes in order to produce an immune response without causing the severe effects of the disease. Attenuated vaccines currently licensed in the United States include measles, mumps, rubella, polio, yellow fever and varicella. Also known as a live vaccine.


A chronic developmental disorder usually diagnosed between 18 and 30 months of age. Symptoms include problems with social interaction and communication as well as repetitive interests and activities. At this time, the cause of autism is not known although many experts believe it to be a genetically based disorder that occurs before birth.

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.


Tiny one-celled organisms present throughout the environment that require a microscope to be seen. While not all bacteria are harmful, some cause disease. Examples of bacterial disease include diphtheriapertussistetanusHaemophilus influenzae and pneumococcus (pneumonia).


Flaws in the collection, analysis or interpretation of research data that lead to incorrect conclusions.

Biological plausibility-

A causal association (or relationship between two factors) is consistent with existing medical knowledge.

Bone marrow

Soft tissue located within bones that produce all blood cells, including the ones that fight infection.

Booster shots

Additional doses of a vaccine needed periodically to "boost" the immune system. For example, the tetanus and diphtheria (Td) vaccine which is recommended for adults every ten years.
Causal association
The presence or absence of a variable (e.g. smoking) is responsible for an increase or decrease in another variable (e.g. cancer). A change in exposure leads to a change in the outcome of interest.

Chronic health condition

A health related state that lasts for a long period of time (e.g. cancer, asthma).

Combination vaccine

Two or more vaccines administered at once in order to reduce the number of shots given. For example, the MMR (measles, mumps, rubella) vaccine.


Capable of spreading disease. Also known as infectious.

Community immunity

Having a large percentage of the population vaccinated in order to prevent the spread of certain infectious diseases. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community. Also known as herd immunity.


A complex series of blood proteins whose action "complements" the work of antibodies. Complement destroys antibody-coated cells, produces inflammation, and regulates immune response.

Conjugate vaccine

The joining together of two compounds (usually a protein and polysaccharide) to increase a vaccine's effectiveness.


Inflammation of the mucous membranes surrounding the eye causing the area to become red and irritated. The membranes may be irritated because of exposure to heat, cold or chemicals. This condition is also caused by viruses, bacteria or allergies.


A condition in a recipient which is likely to result in a life-threatening problem if a vaccine were given.


See Seizure.
Demyelinating disorders
A medical condition where the myelin sheath is damaged. The myelin sheath surrounds nerves and is 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 chronic health condition where the body is unable to produce insulin and properly breakdown sugar (glucose) in the blood. Symptoms include hunger, thirst, excessive urination, dehydration and weight loss. The treatment of diabetes requires daily insulin injections, proper nutrition and regular exercise. Complications can include heart disease, stroke, neuropathy, poor circulation leading to loss of limbs, hearing impairment, vision problems and death.


Sickness, illness or loss of health.
Efficacy rate
A measure used to describe how good a vaccine is at preventing disease.


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


A general term describing brain dysfunction. Examples include encephalitis, meningitis, seizures and head trauma.


The occurrence of disease within a specific geographical area or population that is in excess of what is normally expected.


The continual, low-level presence of disease in a community.

Erythema Multiforme

A medical condition characterized by inflammation of the skin or mucous membranes (including the mouth, throat and eyes). Erthema Multiforme has been reported following infection. Symptoms persist anywhere from 2 days to 4 weeks and include skin lesions, blisters, itching, fatigue, joint pain and fever.


The cause of.


Contact with infectious agents (bacteria or viruses) in a manner that promotes transmission and increases the likelihood of disease.
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 spreads over the body. Muscle paralysis starts in the feet and legs and moves upwards to the arms and hands. Sometimes paralysis can result in the respiratory muscles causing breathing difficulties. Symptoms usually appear over the course of one day and may continue to progress for 3 or 4 days up to 3 or 4 weeks. Recovery begins within 2-4 weeks after the progression stops. While most patients recover, approximately 15%-20% experience persistent symptoms. GBS is fatal in 5% of cases.


Hepatitis B surface antigen.

Herd immunity

See community immunity.

Herpes Zoster

A disease characterized by painful skin lesions that occur mainly on the trunk (back and stomach) of the body but which can also develop on the face and in the mouth. Complications include headache, vomiting, fever and meningitis. Recovery may take up to 5 weeks. Herpes Zoster is caused by the same virus that is responsible for chickenpox. Most people are exposed to this virus during childhood. After the primary infection (chickenpox), the virus becomes dormant, or inactivated. In some people the virus reactivates years, or even decades, later and causes herpes zoster. Also known as the shingles.


The eruption of red marks on the skin that are usually accompanied by itching. This condition can be caused by an allergy (e.g. to food or drugs), stress, infection or physical agents (e.g. heat or cold). Also known as uticaria.


A condition in which the body has an exaggerated response to a substance (e.g. food or drug). Also known as an allergy.


A condition in which the body has a weakened or delayed reaction to a substance.
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. It involves production of protein molecules called antibodies to eliminate foreign organisms that invade the body.


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


The process by which a person or animal becomes protected against a disease. This term is often used interchangeably with vaccination or inoculation.


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.

Inactive vaccine

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

Inapparent infection

The presence of infection without symptoms. Also known as subclinical or asymptomatic infection.


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

Incubation period

The time from contact with infectious agents (bacteria or viruses) to onset of disease.


Capable of spreading disease. Also known as communicable.

Infectious agents

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


Redness, swelling, heat and pain resulting from injury to tissue (parts of the body underneath the skin). Also known as swelling.

Inflammatory Bowel Disease (IBD)

A general term for any disease characterized by inflammation of the bowel. Examples include colitis and Crohn's disease. Symptoms include abdominal pain, diarrhea, fever, loss of appetite and weight loss.

Investigational vaccine

A vaccine that has been approved by the Food and Drug Administration (FDA) for use in clinical trials on 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 eyes and skin. This condition is often a symptom of hepatitis infection.

Live vaccine

A vaccine in which live virus is weakened through chemical or physical processes in order to produce an immune response without causing the severe effects of the disease. Attenuated vaccines currently licensed in the United States include measles, mumps, rubella, polio, yellow fever and varicella. Also known as an attenuated vaccine.

Lock Jaw

See Tetanus.

Lyme disease

A bacterial disease transmitted by infected ticks. Human beings may come into contact with infected ticks during outdoor activities (camping, hiking). Symptoms include fatigue, chills, fever, headache, joint and muscle pain, swollen lymph nodes and a skin rash (in a circular pattern). Long-term problems include arthritis, nervous system abnormalities, irregular heart rhythm and meningitis. Lyme disease can be treated with antibiotics or prevented with the use of a vaccine recently licensed by the Food and Drug Administration.


A transparent, slightly yellow fluid that carries lymphocytes, bathes the body tissues, and drains into the lymphatic vessels.

Lymph Nodes

Small bean-shaped organs of the immune system, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are gathering sites of B, T, and other immune cells.

Lymphatic Vessels

A bodywide network of channels, similar to blood vessels, that transport lymph to the immune organs and into the bloodstream.


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 cells.
A large cell that helps the body defend itself against disease by surrounding and destroying foreign organisms (viruses or bacteria).

Memory Cell

A group of cells that help the body defend itself against disease by remembering prior exposure to specific organisms (e.g. viruses or bacteria). Therefore these cells are able to respond quickly when these organisms repeatedly threaten the body.


Inflammation of the brain and spinal cord 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 and vagina.

Multiple Sclerosis

Multiple sclerosis is a disease of the central nervous system characterized by the destruction of the myelin sheath surrounding neurons, resulting in the formation of "plaques." MS is a progressive and usually fluctuating disease with exacerbations (patients feeling worse) and remissions (patients feeling better) over many decades. Eventually, in most patients, remissions do not reach baseline levels and permanent disability and sometimes death occurs. The cause of MS is unknown. The most widely held hypothesis is that MS occurs in patients with a genetic susceptibility and that some environmental factors "trigger" exacerbations. MS is 3 times more common in women than men, with diagnosis usually made as young adults. Also see demyelinating disorders.


Inflammation of the nerves.


A general term for any dysfunction in the peripheral nervous system. Symptoms include pain, muscle weakness, numbness, loss of coordination and paralysis. This condition may result in permanent disability.

Optic neuritis

A medical condition where vision deteriorates rapidly over hours or days. One or both eyes may be affected. This condition results for the demyelination of optic nerves. In most cases, the cause of optic neuritis is unknown. Patients may regain their vision or be left with permanent impairment. Also see demyelinating disorders.

A complication of mumps infection occurring in males (who are beyond puberty). Symptoms begin 7-10 days after onset of mumps and include inflammation of the testicles, headache, nausea, vomiting, pain and fever. Most patients recover but in rare cases sterility occurs.

Otitis Media

Inflammation of the middle ear caused by a viral or bacterial infection. This condition usually occurs along with an upper respiratory infection. Symptoms include earache, high fever, nausea, vomiting and diarrhea. In addition, hearing loss, facial paralysis and meningitis may result.


Sudden appearance of a disease in a specific geographic area (e.g. neighborhood or community) or population (e.g. adolescents).


An epidemic occurring over a very large area.

Passive immunity

Protection against disease through antibodies produced by another human being or animal. Passive immunity is effective, but protection is generally limited and diminishes over time (usually a few weeks or months). For example, maternal 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 in human beings.


An immune cell that is able to ingest and destroy microbes and other foreign matter.


A substance or treatment that has no effect on human beings.


Inflammation of the lungs characterized by fever, chills, muscle stiffness, chest pain, cough, shortness of breath, rapid heart rate and difficulty breathing.

Polysaccharide vaccines

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, meningococcal disease and Haemophilus Influenzae type b.


A measure of strength.


A condition in a recipient which may result in a life-threatening problem if the vaccine is given, or a condition which could compromise the ability of the vaccine to produce immunity.


The number of disease cases (new and existing) within a population over a given time period.
The isolation of a person or animal who has a disease (or is suspected of having a disease) in order to prevent further spread of the disease.

Reye Syndrome

Encephalopathy (general brain disorder) in children following an acute illness such as influenza or chickenpox. Symptoms include vomiting, agitation and lethargy. This condition may result in coma or death.

Residual Seizure Disorder (RSD)

See Seizure.


The likelihood that an individual will experience a certain event.


A group of viruses that cause diarrhea in children.




The sudden onset of a jerking and staring spell usually caused by fever. Also known as convulsions.


See herpes zoster.

Side Effect

Undesirable reaction resulting from immunization.


A specific version of an organism. Many diseases, including HIV/AIDS and hepatitis, have multiple strains.

Subclinical infection

The presence of infection without symptoms. Also known as inapparent or asymptomatic infection.


Unprotected against disease.
Temporal association
Two or more events that occur around the same time but are unrelated, chance occurrences.


The detection of antibodies in blood through a laboratory test.

Transverse Myelitis

The sudden onset of spinal cord disease. Symptoms include general back pain followed by weakness in the feet and legs that moves upward. There is no cure and many patients are left with permanent disabilities or paralysis. Transverse Myelitis is a demyelinating disorder that may be associated with Multiple Sclerosis (MS). Also see demyelinating disorders.


The eruption of red marks on the skin that are usually accompanied by itching. This condition can be caused by an allergy (e.g. to food or drugs), stress, infection or physical agents (e.g. heat or cold). Also known as hives.


Injection of a killed or weakened infectious organism in order to prevent the disease.


A product that produces immunity therefore protecting the body from the disease. Vaccines are administered through needle injections, by mouth and by aerosol.

Vaccine Adverse Event Reporting System (VAERS)

A database managed by the Centers for Disease Control and Prevention and the Food and Drug Administration. VAERS provides a mechanism for the collection and analysis of adverse events associated with vaccines currently licensed in the United States. Reports to VAERS can be made by the vaccine manufacturer, recipient, their parent/guardian or health care provider. See also CDC VAERS Webpage.

Vaccine Safety Datalink Project (VSD)

In order to increase knowledge about vaccine adverse events, the Centers for Disease Control and Prevention have formed partnerships with four large health Management Organizations (HMOs) to continually evaluate vaccine safety. The project contains data on more than 6 million people. Medical records are monitored for potential adverse events following immunization. The VSD project allows for planned vaccine safety studies as well as timely investigations of hypothesis.


See Chickenpox.


A tiny organism that multiples within cells and causes disease such as chickenpox, measles, mumps, rubella, pertussis and hepatitis. Viruses are not affected by antibiotics, the drugs used to kill bacteria.

Whooping Cough

See Pertussis.