Some parasites alter the behavior of their host in ways that increase their chances of moving on to a new host. These are parasites that need an alternate host in order to complete their life cycle. Toxoplasma gondii , a cat intestinal parasite. The normal intermediate host for T. gondii is the rat. Infected rats lose their natural fear of cats increasing the chance that the parasite will move on to another cat.
Some psychologists claim that infected humans show personality changes as well.
However, infecting humans is a great mistake for the parasite because we are unlikely to eat rats. The relative success of the measures and countermeasures taken by parasites and their host affects the dynamics of their association. For humans, it results in illnesses ranging from acute to chronic. Cause an acute illness of short duration. Contagious for only a brief period.