What are Infectious Diseases? Infectious Diseases. Image Credit: Rost9/Shutterstock.com While many organisms live in the human body without detriment to our health and can often be beneficial, such as is the case with the bacteria that help break down the food we eat and the microbiota in our digestive system that play a role in our immune system, invading organisms are often the cause of disease. The route to infection depends on the pathogen and the nature of the disease it causes. Some are easily passed from person to person while others are transmitted by animals and insects. Some pathogens are introduced into the body via infected food or contaminated water, whereas others may be picked up from certain environments.