Infectious diseases brought on by bacteria, viruses, fungus, or parasites. Our bodies are home to a variety of creatures. In most cases, they're innocuous or even beneficial. However, some microbes can cause sickness under particular circumstances. Some infectious diseases are contagious and can be spread from one person to the next. Insects and other animals can spread several diseases. Others can be contracted by consuming tainted food or drinking tainted water, or by being exposed to organisms in the environment. Fever and exhaustion are common signs and symptoms, which vary depending on the organism that is causing the infection.