Capsid		The protein shell that surrounds a virus particle and encloses the genetic material (DNA or RNA).
Challenge		In laboratory animal vaccine experiments, the deliberate exposure of immunized and unimmunized (control) animals to an infectious agent.
Cavity		In TB, a hollow space or hole within the lung, visible on a chest X-ray, that occurs in people with severe pulmonary TB disease and which may contain many Mycobacterium tuberculosis bacilli.
Clinical trials		Studies in humans in which investigational vaccines are initially evaluated for safety and immunogenicity in small numbers of subjects (Phase I and II) and subsequently evaluated for protective efficacy in large numbers of subjects (Phase III). Trials conducted after a vaccine is licensed to increase knowledge about uncommon adverse events or to study effectiveness in a program setting are referred to as Phase IV trials.
Clinical trial arm		A group of participants in a clinical trial who received one of the study treatments (e.g., an investigational vaccine, an approved vaccine, or a placebo).
Consumption		An alternative name for TB disease, mostly used in the past. People who had consumption were called consumptives.
Control substance		In vaccine trials, an active vaccine or inactive substance (the latter is known as a placebo) given to a group of clinical trial participants for comparison of effects with an investigational vaccine.
Converter		A person whose tuberculin (PPD) skin test result has changed from negative to positive. If conversion occurs within a two-year period after exposure, it is interpreted as recent infection with Mycobacterium tuberculosis and carries an increased risk for progression to the active phase of the disease.
Culture		Microorganisms grown on or in laboratory media (substance containing nutrients) so that they can be identified. A positive culture for Mycobacterium tuberculosis contains tubercle bacilli, whereas a negative culture does not contain detectable tubercle bacilli.
Cytokine		A soluble, hormone-like protein produced by white blood cells that acts as a chemical messenger between cells. Cytokines can stimulate or inhibit the activity of immune cells and may prove useful as immunologic adjuvants.