Thymus - The lymphoid organ in which T lymphocytes are educated, mature and multiply.

T lymphocyte cells - thymus dependent cells, coordinate the cell-mediated immune system.

Transcription - constructing a mRNA molecule using a DNA molecule as a template; results in the transfer of genetic information to the mRNA.

Vaccine - Weakened or destroyed microorganisms used to help provide immunity to a certain disease or microorganism and to stimulate the production of antibodies.

Variant - a variation of a particular strain of virus or infective agent; slightly different in form or function.

Vector - anything capable of moving or transferring genetic material.

Viroid - exotic type of virus which only infects plant cells and is simply a group of naked strings of amino acids with no covering.

Virus - infectious organism that replicates itself in the cells of a host; viruses have a half alive/half dead existence and need a host to grow and reproduce.