Is a white blood cell that swallow up and digest germs, plus dead or dying cells. The macrophages leave behind parts of the invading germs called antigens.
Is an organism that causes disease. Your body is naturally full of microbes. However, these microbes only cause a problem if your immune system is weakened or if they manage to enter a normally sterile part of your body.
The ability of an agent to cause disease after infection, measured as the proportion of persons infected by an agent who then experience clinical disease.