Health refers to a state of physical, mental and social well-being, while illnesses are health problems that affect the body's organs. There are two main types of illnesses - infectious diseases caused by microorganisms like influenza or chicken pox, and non-infectious diseases not caused by microorganisms such as allergies or obesity. Infectious diseases can be caused by bacteria, viruses, protozoans or fungi and spread through personal contact, contaminated objects, animals, food or water. Vaccines help prevent infectious diseases by exposing the body to weakened forms of viruses or bacteria.