Most people think of disease as the result of infection. In fact, disease is any condition that alters normal function. The production of completely disease-free flocks, though, is not economically possible. To minimize losses, rather than simply treating the disease, we must focus on the factors that lead to the disease.
Most pathogens are parasitic and survive at the expense of the host.
Many have a predilection for certain types of tissue. They also vary in the way they invade and affect the cells of the body. First line of defense against the invasion of pathogens is the skin and mucous membranes, specialized surfaces that line the respiratory and intestinal tracts. Besides acting as a mechanical barrier, the mucous membranes may have specialized surfaces. They may produce cleansing fluids or beat rhythmically to move the invaders to the outside of the body.