Macrophage - contains lysosomes full of special enzymes and chemicals, which allows it to ingest and digest dangerous microbes

Memory cells - keep the body prepared for the next struggle with antigens that have already been encountered.

mRNA (messenger RNA) - used as the carrier of genetic codes and information directly from DNA to cell structures.

Mutation - a rearrangement of genes or change in base pairs so they produce different effects within their environment.

Nanometer - One-millionth of a millimeter.

Nucleic acid - an organic compound made up of a phosphoric acid, a carbohydrate and a base of purine or pyrimidine; formed in helical chains.

Nucleus - a cellular organelle that is the essential control mechanism for cell function; contains the DNA and genetic material.

Origin - location where the process of replication in a nucleic acid begins.

Parasite - an organism living in or on another organism that depends on its host for existence or support and gives nothing in return.

Pathogenesis - the growth and development of a disease.

Prion - an infectious crystallizing protein, which affects the brain.

Proteins - strands of amino acids which make the enzymes and structures needed for cells to grow and function properly.

RNA (ribonucleic acid) - strings of organic material, obtained from DNA, which store the proper instructions necessary to produce amino acids.

Receptor - a structure on a cell which joins with proteins to produce changes in cellular function.

Replication - the action or process of reproducing exact copies of one's self.

Retrovirus - An RNA virus that converts its RNA into DNA by means of the enzyme reverse transcriptase and integrates itself directly into the host's DNA.

Reverse transciptase - special enzyme which allows retroviruses to translate genetic code from RNA to DNA.

Ribosome - the spherical structure that assembles proteins after being fed the genetic instructions by mRNA.

Spleen - an organ that produces lymphocytes, filters the blood, and stores blood cells

Strain - a specific type, quality, or disposition of a material.

Suppressor T cells - monitor and adjust antibody levels in the body; act as suppressors to counteract the Helper and Killer T cells at the end of infections.