Parasites organism that grows, feeds, and is sheltered on or in a different organism and that does not contribute to the survival of its host. Virus ultimate micro-parasite smaller than bacteria; neither cells not organisms; can only reproduce within their host. Macro-parasites composed of many cells; cycles through transmission stages (eggs and larvae) which pass into the external environment. Transmission - movement of a parasite from host to host. Incubation period - interval of time required for development of a disease. Latent period -seemingly inactive period between exposure to an infection and subsequent illness. Parasite virulence-capacity of a parasite to cause disease. Zoonotic infections - animal infections that can be transmitted to humans. Host immunity,genetics,nutrition, agent- biological, physical chemical, psychosocial, rate of growth, persistence. Modes of disease transmission- - direct transmission - from person to person. Indirect through a common route / vector, e. g. contaminated air / water, mosquito.