Introduction to Contagion


Contagion and contagiousness refers to how easily the spread of an infectious disease is possible from one person to another. Other words for contagion include "infection", "infectiousness", "communicable", "communicability", "transmission" or "transmissability". Common terms used are whether you can "catch" a disease from someone who already has it.

Inheritance and contagiousness unrelated: Contagiousness has nothing to do with genetics or inheriting diseases from parents. Genetic inheritance and contagion are not the same thing. You are born with your genes in your DNA, and they cannot be transferred. Genetic conditions are inherited from parents, and you have them from birth. You cannot acquire a genetic condition after birth. Contagiousness refers to the transfer of an infectious agent (e.g. virus or bacteria) between people. For an overview of genetics and inheritance, see Introduction to Genetics.

Diseases that are contagious: There are many types of diseases that are caused by an infectious agent and are therefore contagious to other people. The main types of diseases that are generally regarded as contagious include:

  • Viruses: Numerous viruses like the common cold and flu are contagious very easily.
  • Bacterial diseases: Most types of bacteria are contagious quite easily.
  • Fungal conditions: Various types of fungus can transfer disease between people.
  • Parasitic conditions: Certain types of parasites can be spread by people.
  • Worm conditions: Some types of infectious worms can infect other people.
  • Prion diseases: The transfer of infectious proteins is possible for this rare type of disease, but usually only by direct exposure to brain tissue (e.g. by performing brain surgery or autopsy).

Non-contagious diseases: Many types of disease are non-contagious. Obviously physical conditions like an ankle sprain during sports is not contagious. Here are some other examples of conditions not normally regarded as contagious:

  • Genetic diseases: You are born with your DNA and cannot catch bad DNA from another person.
  • Cancers: You cannot catch cancer from someone who has it. It is not contagious by saliva, sexual activity, body fluids, or even by blood. The only extremely rare ways to catch cancer include organ transplants and very rare cases of fetus-to-mother or mother-to-fetus contagion.
  • Autoimmune diseases: Transmission of autoimmune diseases does not occur from person to person. Although these diseases are believed to be caused by white blood cells in the blood, autoimmune diseases are not even believed contagious by blood or body fluids (presumably a healthy person's immune system simply absorbs and overwhelms the misbehaving new cells). The only examples of contagion of autoimmune diseases are laboratory transfers by immune white blood cell transfer in immunosuppressed mice, and rare mother-to-fetus contagion of some autoimmune diseases such as lupus (see neonatal lupus) and myasthenia gravis. The situation is not entirely clear with organ transplants, since some immune system factors can occur, such as Graft-versus-Host disease.
  • Mental conditions: You cannot catch depression, anxiety disorders or other mental conditions from other people. Perhaps you can be affected by talking to people, and how they react, but mental diseases are not infectious.
  • Metabolic conditions: various diseases arise from changes to the internal way the body's metabolism is working. Some examples include diabetes, obesity, high cholesterol, atherosclerosis, and hypertension.
  • Hormonal conditions: The inner workings of the body's hormones can have various disorders, but these are not contagious unless there is an underlying infectious problem such as a virus actually causing the problem.
  • Congenital conditions: This term means any condition that you are born with. All genetic diseases are congenital, but the group of congenital conditions also includes physical abnormalities that you are born with, such as congenital heart defects. These conditions are obviously not contagious.
  • Malnutrition disorders: Obviously you cannot catch a disease that is caused by a dietary deficiency or general malnutrition.
  • Chemical or toxic disorders: Obviously a disease caused by exposure to a chemical, toxin, radiation, or other physical exposure is not a disease that will spread to others.

Methods of Contagion

There are many ways to catch an infectious disease. Some are spread by droplets from sneezing, others by insects, others by sexual activity, and others from blood transfusions or intravenous needle usage.

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