Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They're normally harmless or even helpful. But under certain conditions, some organisms may cause disease. Some infectious diseases can be 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. Frequent and thorough hand-washing also helps protect you from most infectious diseases.
In the development of a disease (pathogenesis) more is involved than merely exposure to a causative agent. A room full of people may be exposed to a sufferer from a common cold, but only one or two may later develop a cold. Many host factors determine whether the agent will induce disease or not. Thus, in the pathogenesis of disease, the resistance, immunity, age, and nutritional state of the person exposed, as well as virulence or toxicity of the agent and the level of exposure, all play a role in determining whether disease develops.
Certain human diseases result from mutations in the genetic complement (genome) contained in the DNA of chromosomes. A gene is a discrete linear sequence of nucleotide bases of the DNA that codes for, or directs, the synthesis of a protein; there are an estimated 20,000 to 25,000 genes in the human genome. Proteins, many of which are enzymes, carry out all cellular functions.
