Those parasitic organisms capable of causing disease are called pathogens. Disease is any change in the host from a healthy to an unhealthy, abnormal state in which part or all of the host’s body is not properly adjusted or capable of carrying on its normal functions.

For a parasitic organism to cause disease, it must be transmitted to a suitable host, attach to and/or colonize the host, grow and multiply within or on the host, and interfere with or impair the normal physiological activities of the host. When a parasitic organism is growing and multiplying within or on a host, the host is said to have an infection.

The host’s ability to resist infection depends on a continuous defense against parasitic invasion.

Resistance arises from both innate and acquired body defense mechanisms. The innate, general, or nonspecific immune mechanisms are those with which a host is genetically endowed and include general, physical, chemical, and biological barriers.

Acquired resistance mechanisms are those that the host acquires on contact with either the parasitic organism or its products.