Malaise - A general feeling of being unwell; feeling may be accompanied by identifiable physical discomfort and may indicate the presence of disease.

Marker - An antigen or antibody used to indicate the status of disease or recovery.

Mediate - Act indirectly to affect a result.

Morbidity - The state of being diseased.

Myalgia - Pain in the muscles.

NANB (non-A non-B) hepatitis - Viral hepatitis caused by viruses other than A or B.

Parenteral - Used to refer to entering the body subcutaneously (under the skin), intramuscularly (into a muscle), or intravenously (into a blood vessel); may refer to any other means whereby the organism reaches the bloodstream directly.

Percutaneous - Through the skin.

Permucosal - Through the mucous membranes.

Picornavirus - A virus family consisting of small RNA viruses, HAV belongs to the picornavirus family.

Prevalence (of a disease) - The percentage of a population that is affected with a particular disease at a given time.

Prophylaxis - The prevention of disease.

Replication - Production of a copy or image of itself; encompasses the steps that a virus goes through to reproduce (the term "duplication" is used for cells that split in two to reproduce themselves).

RNA (ribonucleic acid) - A substance formed in the cell nucleus, under the control of DNA; transfers genetic code into the cell for the synthesis of proteins.

Sequela - An illness or condition that follows as a consequence of another disease (plural form: sequelae).

Seroconversion - An immune response that is characterized by a conversion from the absence of a specific antibody to the presence of that specific antibody in a patient or the disappearance of an antigen followed by the appearance of its corresponding antibody.

Serological - Pertaining to antigen-antibody reactions in vitro.

Seronegativity - Blood serum showing a negative result to a specific test.

Seropositivity - Blood serum showing a positive result to a specific test.

Serum hepatitis - Old term for hepatitis B.

Superinfection - A condition whereby an already infected individual becomes infected with a virus different from the original infecting agent.

Subclinical - Without clinical manifestations or symptoms.

Syndrome - A set or collection of symptoms and signs that occur together.

Viral load - The amount or concentration of virus in the circulation.

Virology - The study of viruses.

Virus - A collection of proteins and nucleic acids capable of infecting other cells; viruses can multiply only within the cells that they are infecting.