All of the preceding physiology we have discussed is worthless if the animal cannot resist disease. Anything that causes animal disease is referred to as a pathogen.
All animals have non-specific barriers to pathogens.
These non-specific defenses are always available. They are found in almost all animals, including many invertebrates. For example, both human skin and an insect's cuticle are barriers to pathogens. Lysozyme is present in your tears, among other fluids, and also in insect hemolymph. Most invertebrates, like vertebrates, also have phagocytic blood cells that eat pathogens, if they are small enough.