Commensalism - One benefits while the other, neither benefits nor is harmed. Synergism - Together both can accomplish what neither can achieve alone. Parasitism - One benefits and causes harm to the other. Organism that live with a host without causing an immune or allergic response is called a Colonizer. Organisms that cause disease are pathogenic.
Virulence is a quantitative measure of pathogenicity.
Loss or reduction of virulence is called Attenuation. Organisms can be treated with agents to achieve attenuation in order to be used as vaccines. Some individuals can be infected by a pathogen, but show no symptoms and are called Carriers. Focal - Original site of a disease. Systemic - Organisms spreading through the body.