Disease is a pathophysiological state of disruption or imbalance of the body’s homeostatic systems. The term is used to classify conditions that interfere with the chemical, physical and functional processes of life, and may be caused by infectious organisms such as bacteria, viruses, fungi, multicellular organisms or aberrant proteins known as prions. Diseases are characterized by their ability to cause significant impairment of the host’s normal functioning and by their potential for spreading to others.
Infectious diseases are illnesses that occur when tiny living beings (such as fungi, rickettsia, bacteria, viruses and protozoans) get inside the body and start multiplying. Some of these microorganisms spread directly from one person to another via blood or secretions (such as sneezing and coughing), while others spread by infecting nearby cells and then exiting the host to infect new hosts.
There are other diseases that don’t spread, or that only spread between closely related individuals. These include genetic diseases, acquired diseases and iatrogenic diseases. A genetic disease is one that is caused by a mutation in the DNA.
An acquired disease is one that was not present at birth, or congenital, and it is often a result of a chemical or physical injury. An iatrogenic disease is one that occurs as a result of medical treatment, or may be due to a side effect of a drug. An organic disease is a condition that affects the body’s structural and functional processes, and can involve either physical or psychological changes.