Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms that live in and on our bodies are normally harmless and may even be helpful. But under certain conditions, some organisms may cause disease. Some infectious diseases are passed from person to person. Some are transmitted by insects or other animals. And you may get others by consuming contaminated food or water or being exposed to organisms in the environment. Signs and symptoms vary depending on the organism causing the infection, but often include fever and fatigue. Mild infections may respond to rest and home remedies, while some life-threatening infections may need hospitalization.