Vaccine: A preventative treatment of prepared microorganisms by means of either killed (dead) microorganisms,  living (i.e. weakened / attenuated) microorganisms. or inactivated bacterial toxins. Vaccines are administered to induce the immune response and provide protection against a pathogen or a toxin.

​Vein:​ Blood vessel which returns blood to the heart. Adapted to minimise the resistance to blood flow despite low pressure. Has; thin walls, large lumen & valves. Relies heavily on action of surrounding skeletal muscles.

Ventricle: Lower chamber of heart, contracts to force blood into aorta or pulmonary artery.

Ventricular Systole: Contraction of the ventricle.

Vessel element: one of the different types of cell that makes up xylem tissue.

​Virion: Complete virus particle, the simplest consists of just a protein capsid surrounding a single nucleic acid molecule (either DNA or RNA).

Virology: The branch of microbiology that is concerned with the study (structure, function, classification, epidemiology etc.) of viruses and viral diseases.

Virulence: The level of pathogenicity as indicated by fatality rates (cases) and/or the ability to invade host tissues and cause disease. So, in simple terms: Virulence is how pathogenic an organism is.

Virus: A simple acellular infectious agent. An obligate intracellular parasite”. A complete virus particle is called a virion consisting of a protein coat and a nucleic acid genome. Viruses lack independent metabolism and replication only takes place within living host cells.