Glossary Please select an letter from the A-Z list or search # A B C D E F G H I J K L M N O P Q R S T U V W X Y Z vaccine A substance that contains antigenic components from an infectious organism. By stimulating an immune response (but not disease), it protects against subsequent infection by that organism, or may direct an immune response against an established infection or cancer. variceal hemorrhaging Bleeding caused by bursting varices (stretched veins). varices Stretched veins which may burst and cause severe bleeding; a complication of cirrhosis. vertical transmission Transmission, for example of HIV, from mother-to-baby. viraemia The presence of virus in the blood. viral load Measurement of the amount of virus in a sample. For hepatitis C, see ‘RNA’. For hepatitis B, see ‘DNA’. virion A virus particle existing freely outside a host cell. virologic response The effect of treatment on viral load. virulence The power of bacteria or viruses to cause a disease. Different strains of the same micro-organism can vary in virulence. virus A microscopic germ which reproduces within the living cells of the organism it infects. visceral Of or pertaining to the internal organs. vomiting Being sick. vulva The external female genitals.