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.

• Droplet infection (including sneezing): Various diseases are spread by droplets that are inhaled into the respiratory system. See also diseases contagious from droplet.
• Airborne infection: The air can carry droplets, including droplet infection from sneezing or coughing. But other types of airborne infection are possible such as in Legionnaires' disease. See also airborne diseases.
• Waterborne contagion: Various diseases can be spread by contaminated water sources. The act of drinking water can also spread disease by saliva. See water and saliva. See also waterborne diseases, contaminated water, and diseases contagious from contaminated water.
  • Swimming: catching the disease by swimming, including a swimming pool, or outdoors swimming activities. See also water activities.
  • Swimming pool contagion: whether the condition can be caught from swimming in an ordinary chlorinated or salt water swimming pool.
• Food contagion: whether the condition can be spread by contamination of food. See food and diseases contagious from food.
• Physical contact: shaking hands, touching another person. Numerous conditions such as common cold or flu can be spread by the hands. See also physical contact and diseases contagious from physical contact.
• Saliva (including kissing): Various diseases can be spread by saliva, particularly from kissing. Some examples include common cold, flu, mononucleosis, cold sores, meningococcal disease, and various others. See also saliva, diseases contagious from kissing, and diseases contagious from saliva.
• Surface contagion: whether the disease can be picked up by touching a surface that has been touched or sneezed on by an infected person. See also surfaces and diseases contagious from surfaces.
• Clothing or bedding: whether the disease is contagious by the clothing, bedding, towels, or other items handled by an infected person. See also clothing, diseases contagious from clothing, diseases contagious from towels, and diseases contagious from bedding.
• Fecal-oral contagion: Some conditions, particularly certain worm conditions are spread by infected feces contaminating the hands, which are then transfered to the mouth. See also feces and diseases contagious from feces.
• Sexual activity: Whether the disease can be spread by sexual behavior or sexual fluids. See also contagion of sexually transmitted diseases, diseases contagious from sex, and sex.
  • Vaginal sex: Whether the disease can be spread by unprotected vaginal sexual intercourse; see diseases contagious from vaginal sex.
  • Oral sex: Whether the disease can be spread by unprotected oral sex; see diseases contagious from oral sex.
  • Anal sex: Whether the disease can be spread by unprotected anal sexual intercourse; see diseases contagious from anal sex.
  • Safe sex: contagion even when a condom is worn during sex; see diseases contagious from safe sex.
• Blood contagion: Certain types of disease are spread through blood fluid transfer from one person to another. This blood transfer can occur through intravenous needle usage, blood transfusions (though safe in modern developed countries), and sometimes by casual blood transfer across wounds. Organ transplants are also a special case of this type of contagion with additional issues. See also diseases contagious from blood, blood and transplants.
  • Intravenous needles: The use of intravenous needles for illicit drug use is one of the most common methods of blood contagion, now that transplants are safer. See intravenous drug use.
  • Blood transfusions: Although modern blood transfusions do not transfer disease, there are still risks in undeveloped areas. See transfusions.
  • Blood product transfusions: similar issues exist for transfusions of blood products rather than whole blood; see transfusions.
  • Organ transplants: Whether an implanted organ can cause this disease in the person receiving the organ if the donor had the disease. See also transplants.
• Mother-to-child contagion: If the mother has a disease, it is often likely that the child is at risk of contagion.
  • Mother-to-fetus contagion: a number of maternal conditions can be passed to a fetus during pregnancy. The child is then born with a congenital infection. Some examples include syphilis (see congenital syphilis), lupus (see congenital lupus), myasthenia gravis, toxoplasmosis, Hepatitis C and some others. See also mother-to-fetus transmission.
  • Fetus-to-mother contagion: extremely rarely, a fetus can get a disease first, and then pass it to the mother through the placenta.
  • Mother-to-newborn childbirth contagion: certain conditions can be passed to the newborn during the event of childbirth. The newborn is exposed to any viruses or bacteria that are present in the vaginal canal, and some of these can cause disease. See also childbirth transmission.
  • Breastfeeding contagion: a nursing infant can become infected with some conditions from the mother, either through the breast milk or by the closeness of the act of breastfeeding. See also breastfeeding, breastfeeding, breast milk, and diseases contagious from breast milk.
• Vector-borne contagion: the transfer of the disease by a "vector", usually an insect, typically from a "reservoir" of the disease, usually in some animal or bird host that is bitten by the vector insect.
  • Insects: several types of insects can spread insect-borne diseases, including mosquitos, ticks, flies, fleas, and lice. See also Insect-Borne Diseases and insects.
  • Mosquitos: Various diseases are spread by mosquito bites such as malaria, dengue, and various other mosquito-borne conditions. See also mosquitos and mosquito-borne conditions.
  • Ticks: Various diseases are spread by tick bites including typhus, Lyme disease, and various other tick-borne conditions. See also ticks and tick-borne conditions.
  • Flies: see also flies and fly-borne conditions.
  • Fleas: see also fleas and Flea-borne diseases.
  • Lice: These parasites can be a disease in themselves (e.g. scabies, pubic lice, head lice) but can also spread other bacterial diseases such as louse-borne relapsing fever.
• Animals: Various animals can spread their diseases to humans; see animals.
  • Rodents (Rats, Mice): There are various rodent-borne conditions that can affect humans. See also rodents and rodent-borne conditions.
  • Cats: see cats and diseases contagious from cats.
  • Dogs: see dogs and diseases contagious from dogs.
  • Pigs: see pigs and diseases contagious from pigs.
• Plants: certain plants can spread disease; see plants.

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