Pandemic
A disease occurring over a wide geographic area and affecting a very high proportion of the population. This term is often used to describe large outbreaks of influenza that occur worldwide and cause a high rate of death.
Pathogen
An organism that can cause disease, such as a bacterium or a virus.
Pathogenic
Capable of causing disease.
Pathogenesis
The mechanism by which a certain agent causes disease.
Peptide
Short chain of linked amino acids. Amino acids are the building blocks of proteins.
Phase I clinical trial
The first stage in testing new drugs or vaccines in humans. Phase I trials are performed on small numbers of people and are designed primarily to test the safety of the new drug and to obtain information about dosages. Drugs that pass phase I trials go on to trials that determine effectiveness and possible side effects and are tested on larger groups of people. If safety and effectiveness are demonstrated, the drugs or vaccines may become approved as treatments.
Placebo
An inert substance used in a controlled experiment to test the efficacy of another substance, such as a drug or a vaccine.
Plasmid
A circular segment of DNA that encodes a separate set of genes than those present in chromosomes. Plasmids are most often found in bacteria, but they are also useful to scientists as vectors.