Infectious diseases, sometimes called contagious diseases, are illnesses caused by organisms such as bacteria, viruses, fungi or parasites. Some infectious diseases can be passed from person to person. Some are transmitted by bites from insects or animals. And others are acquired by ingesting contaminated food or water or being exposed to organisms in the environment.
Common infectious diseases include: influenza, whooping cough, chlamydia, gonorrhea, acquired immunodeficiency syndrome (AIDS), salmonellosis, hepatitis A and B, shigellosis, tuberculosis (TB), primary and secondary syphilis, and Lyme disease.