Microorganisms are tiny living creatures, such as bacteria, fungi, and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the hundreds of thousands of species of microorganisms invade, multiply, and cause illness in animals and people.
Many microorganisms live on the skin and in the mouth, upper airways, and intestines without causing disease. Whether a microorganism lives harmlessly in or on a pet, or invades and causes disease depends on both the nature of the microorganism and the state of the pet’s natural defenses.
This chapter provides a general introduction to the causes, signs, diagnosis, and treatment of infections.