Acquired immunity

A type of immunity that creates immunological memory after an initial response to a specific pathogen, and leads to an enhanced response to future encounters with that pathogen. Unlike the innate immune system, which is pre-programmed to react to common broad categories of pathogen, the adaptive immune system is highly specific to each particular pathogen the body has encountered. The adaptive immune system includes both humoral immunity components and cell-mediated immunity components and destroys invading pathogens. Antibodies are a critical part of the adaptive immune system. Adaptive immunity can provide long-lasting protection, sometimes for the person’s entire lifetime.


The relative capacity of a pathogen to overcome body defenses and elicit symptoms.


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

Virus Vector

A form of a virus used to deliver genetic material into a cell.

VLP vaccine

Vaccine made from virus like particle (VLP). VLPs contain repetitive, high density displays of viral surface proteins that present conformational viral epitopes that can elicit strong T cell and B cell immune responses. Since VLPs cannot replicate, they provide a safer alternative to attenuated viruses. Vaccines for Hepatitis B and human papillomavirus, are developed and FDA-approved.

Vulnerable Subjects

Groups of people whose range of options is severely limited, who may be subjected to coercion or who may be compromised in their ability to give informed consent to receive medical or surgical treatments or to participate in research. This includes pregnant women and fetuses, minors, prisoners, persons with diminished mental capacity, and those who are educationally or economically disadvantaged.


Cowpox is an infectious disease caused by the cowpox virus. Cowpox is similar to, but much milder than, the highly contagious and often deadly smallpox disease. Its close resemblance to the mild form of smallpox and the observation that dairy farmers were immune to smallpox inspired the modern smallpox vaccine, created and administered by English physician Edward Jenner.

Washout period

The length of time that someone enrolled in a trial must not receive any treatment before receiving the trial’s experimental intervention.

Virus like particle (VLP)

Nanoscale structures made up of assembled viral proteins that lack viral genetic material and are therefore non-infectious.

Whooping cough

Bacterial infectious disease caused by bacterium Bordetella pertussis, marked by a convulsive spasmodic cough, sometimes followed by a crowing intake of breath. Also known as pertussis.


Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. Some species of yeast are opportunistic pathogens that can cause infection in people with compromised immune systems. Candida and Cryptococcus are significant pathogens of immunocompromised people.

Yellow fever

The disease caused by the yellow fever virus, characterized by fever, chills, loss of appetite, nausea, muscle pains and headaches. It is spread by the bite of an infected mosquito.


An infectious disease that is transmitted between species from animals to humans.


Infectious disease usually caused by Mycobacterium tuberculosis. Tuberculosis generally affects the lungs, but it can also affect other parts of the body, including pleura, meninges, bones, etc.


Characterized by small fluid-containing elevations of the skin (blisters).

Type I error

A false positive, is the rejection of the null hypothesis when it is actually true.


A branch of medicine that focuses on animal (non-human) care.

Type II error

A false negative, is the failure to reject a null hypothesis that is actually false.

Viral shedding

Expulsion and release of virus progeny following successful reproduction during a host cell infection. Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruses may begin to leave the cell. This can also occur in instances of infection caused by some attenuated vaccines. A human with a viral disease can be contagious if they are shedding virus particles, even if they are unaware of doing so.


A disease caused by Salmonella enterica serotype Typhi bacteria, also called Salmonella typhi. Symptoms vary from mild to severe, and often there is a gradual onset of a high fever over several days. This is commonly accompanied by weakness, abdominal pain, constipation, headaches, and mild vomiting.


The presence of a virus in the blood.


The degree to which overt adverse effects can be tolerated by the subject or patient.

Typhoid Fever

Typhoid fever is a life-threatening illness caused by the bacterium Salmonella Typhi. Persons with typhoid fever carry the bacteria in their bloodstream and intestinal tract.

Toxic Hepatitis

Impairment of liver function caused by taking various drugs.


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 a food or drug), stress, infection, or physical agents such as heat or cold. Also known as hives.


The degree to which a chemical substance or a particular mixture of substances can damage an organism.


The physical act of administering any vaccine.


The scientific study of the characteristics and effects of poisons.

Vaccination schedule

A series of vaccinations, including the timing of all doses, which may be either recommended or compulsory, depending on the country of residence.

Transverse Myelitis

Sudden-onset inflammation of the spinal cord. 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). See demyelinating disorders.


A poisonous substance, especially one produced by bacteria, that causes disease.


A substance used to stimulate immunity to a particular infectious disease or pathogen. A suspension of live (usually attenuated) or inactivated microorganisms (e.g., bacteria or viruses), highly defined antigens, or genetic material of the administered to induce immunity and prevent infectious diseases and their sequelae.

Viral vector vaccine

A vaccine that uses a modified, harmless fraction of a different virus (a vector virus) associated with an antigen that can induce production of important instructions that are delivered to the body’s cells.


An inactivated toxin (usually an exotoxin) whose toxicity has been suppressed either by chemical (formalin) or heat treatment, while other properties, typically immunogenicity, are maintained.

Vaccine Safety Datalink Project (VSD)

A collaboration between CDC and eight large Health Management Organizations (HMOs) to continually evaluate vaccine safety and increase knowledge of vaccine adverse events. Medical records of more than 6 million people are monitored for potential adverse events following vaccination, which supports vaccine safety studies and enables timely investigations.


The process by which a cell makes an RNA copy of a piece of DNA.


A virus related to the smallpox and cowpox viruses, which is used in smallpox vaccine.


The process by which a virus transfers genetic material from one bacterium to another.


The expected value of the squared deviation from the mean of a random variable. In other words, a measurement of the spread between numbers in a data set.


A process by which foreign nucleic acids are delivered into a eukaryotic cell to modify the host cell’s genetic makeup.


A subtype of a microorganism that is genetically distinct from a main strain, but not sufficiently different to be termed a distinct strain.

Transgenic mouse

Mice that have been artificially modified at a genetic level to include a foreign sequence.


An acute contagious disease characterized by papular and vesicular lesions. Also known as chickenpox.

Transmission Electron Microscopy (TEM)

Microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen.


An acute, highly infectious, often fatal disease caused by a poxvirus and characterized by high fever and aches with subsequent widespread eruption of pimples that blister, produce pus, and form pockmarks. Also known as smallpox.


The fact of living things turning towards or away from something, for example light.


A vector is any particle used as a vehicle to artificially carry a foreign nucleic sequence (usually DNA) into another cell, where it can be replicated and/or expressed. The four major types of vectors are plasmids, viral vectors, cosmids, and artificial chromosomes.


The reactant which is consumed during a catalytic or enzymatic reaction.


Of or relating to the treatment of illness.

Subunit vaccine

A vaccine that contains purified parts of the pathogen that are antigenic, or necessary to elicit a protective immune response.

Therapeutic Index (TI)

Range of doses at which a medication is effective without unacceptable adverse events.

Sudden Infant Death Syndrome (SIDS)

The sudden and unexpected death of a healthy infant younger than 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. Also known as crib death or cot death.


The detection of antibodies in blood through a laboratory test, the concentration of antibodies detected in such a test.

Sulfotransferase (SULT)

A family of phase II enzymes transferring a sulfate group from 3′-phosphoadenylyl sulfate to the hydroxyl group of an acceptor. It plays important role in metabolism of various drugs.


A method of finding exactly how much of a substance there is in a solution by gradually adding measured amounts of another substance that reacts to it.

Superiority Trial

A trial aims to show that one treatment is clinically better than the other by demonstrating superiority of the test agent over placebo.


A metabolically active assembly of phospholipids and surfactant-specific proteins that is essential for normal lung mechanic.

Synergistic Effect

The situation when the interaction between two or more drugs causes the total effect of the drugs to be greater than the sum of the individual effects of each drug.

Surrogate Variable

A variable that can be measured (or is easy to measure) that is used in place of one that cannot be measured (or is difficult to measure).


A disease that is caused by the bacterium Treponema pallidum and spread through sexual intercourse. The primary stage classically presents with a single painless skin ulceration. In secondary syphilis, a diffuse rash occurs, which frequently involves the palms of the hands and soles of the feet. In tertiary syphilis, there are gummas, neurological problems, or heart symptoms.


The ongoing evaluation of an individual who has an increased risk of developing a disease or who has a disease that appears to be clinically stable or not progressing.

Systemic Circulation

The circuit of vessels supplying oxygenated blood to and returning deoxygenated blood from the tissues of the body, as distinguished from the pulmonary circulation.

Systemic lupus erythematosus

A disease characterized by inflammation of the connective tissue that supports and connects all parts of the body. Chronic swelling of the connective tissue causes damage to the skin, joints, kidneys, nervous system, and mucous membranes. The disease begins with fever, joint pain, and fatigue. Additional symptoms continue to develop over the years, including nausea, fatigue, weight loss, arthritis, headaches, and epilepsy. Problems with heart, lung, and kidney function may also result. This condition is diagnosed most frequently in young women but occurs in children as well.

T cell

A type of white blood cells of the immune system that plays a central role in the adaptive immune response. T cells are born from hematopoietic stem cells, and then migrate to the thymus gland to mature. T cell has two major subtypes: CD8+ cytotoxic T cells and CD4+ helper T cells. Cytotoxic T cells mainly function by directly killing virus-infected cells, as well as cancer cells. Helper T cells mainly function by further activating memory B cells and cytotoxic T cells, which leads to a larger immune response.

T helper cell

A type of T cell that plays an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considered essential in B cell antibody class switching, breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils.

Subclinical infection

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

Tangential flow filtration (TFF)

A type of filtration used in biochemical engineering that biomolecules are separated and purified while feed flow travels tangentially across the surface of the filter.


A person who participates in a clinical trial and is subject to the application of investigational drugs, medical devices, or control devices.

Td vaccine

A vaccine against tetanus (T), diphtheria (d). Lower-case “d” means this vaccine use smaller doses.

Subject Identification Code

A unique identification code assigned by the principal investigator to each subject to protect their identity. This code is used by the principal investigator to report adverse events or other clinical trial-related data instead of the subject’s name.

Tdap vaccine

A vaccine against tetanus (T), diphtheria (d), and pertussis (ap). Lower-case “d” and “p” means this vaccine use smaller doses. ‘a’ stands for ‘acellular’, meaning it contains only part of the pertussis bacteria instead of the whole bacteria.


A bacterial infection caused by Clostridium tetani. The bacteria generally enter through a break in the skin, such as a cut or puncture wound caused by a contaminated object. They produce toxins that interfere with normal muscle contractions, causing characteristic muscle spasm in those who are infected.

Side Effect

Refers to all unintended effects that occur when a drug is administered at normal dosages, including adverse experiences (AE), signals, and adverse drug reactions (ADR).

Silence (Gene)

Mutations in DNA that do not have an observable effect on the organism’s phenotype. Mutations that cause the altered codon to produce an amino acid with similar functionality (e.g. a mutation producing leucine instead of isoleucine) are often classified as silent.

Small biologic

Small-sized biologics, such as antibodies that are typically composed of 2,000 to 3,000 atoms.


An acute, highly infectious, often fatal disease caused by a poxvirus and characterized by high fever and aches with subsequent widespread eruption of pimples that blister, produce pus, and form pockmarks. Also known as variola.


Probability of a negative test result, conditioned on the individual truly being negative.


Study of the absorption and emission of light and other radiation by matter, as related to the dependence of these processes on the wavelength of the radiation.


Act of sorting data, people, and objects into distinct groups or layers.

Spike protein

A protein that forms a large structure known as a spike or peplomer projecting from the surface of an enveloped virus.  The proteins are usually glycoproteins that form dimers or trimers. Spikes typically have a role in viral entry. They may interact with cell-surface receptors located on the host cell and may have hemagglutinating activity as a result, or in other cases they may be enzymes.


A person, company, institution, group, or organization that oversees or pays for a clinical trial and collects and analyzes the data.


Probability of a positive test result, conditioned on the individual truly being positive.


A point in a project or plan at which development can be examined and any important changes or decisions relating to costs, resources, profits, etc.


A condition which is the consequence of a previous disease or injury.

Standard treatment

Treatment that is accepted by medical experts as a proper treatment for a certain type of disease and that is widely used by healthcare professionals. In many scientific studies, the control group receives the standard treatment rather than a placebo while a treatment group receives the experimental treatment.

Serious Adverse Event (SAE)

Adverse event associated with use of medical product, which includes death, life threatening, hospitaliation, disability and/or permanent damage, congenital anomaly and/or birth defect.

Stem cell

Undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell.


Development of antibodies in the blood of an individual who previously did not have detectable antibodies.


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


A way of grouping cells or microorganisms, such as bacteria or viruses, based on the antigens or other molecules found on their surfaces.

Streptococcus pneumoniae

A Gram-positive, spherical bacteria, alpha-hemolytic member of the genus Streptococcus. They are usually found in pairs (diplococci) and do not form spores and are non motile. It is a major cause of pneumonia, and recognized as a significant human pathogenic bacterium.


The fluid and solvent component of blood. Serum contains all proteins except clotting factors (involved in blood clotting), including all electrolytes, antibodies, antigens, hormones; and any exogenous substances (e.g., drugs, microorganisms).

Severe Combined immune Deficiency (SCID)

A group of rare, life-threatening disorders caused by at least 15 different single gene defects that result in profound deficiencies in T- and B- lymphocyte function.


An infection of the intestines caused by Shigella bacteria. Symptoms generally start one to two days after exposure and include diarrhea, fever, abdominal pain, and feeling the need to pass stools even when the bowels are empty. The diarrhea may be bloody.


A disease characterized by painful skin lesions that occur mainly on the trunk (back and stomach) of the body but can also develop on the face and in the mouth. Complications include headache, vomiting, fever, and meningitis. Recovery can take up to 5 weeks. Herpes Zoster is caused by the same virus 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 herpes zoster.


A prescribed course of medical treatment.


(Chemistry) A chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions.