Contagion is the transmission of an infectious disease directly or indirectly.
Direct and indirect contagion
In the case of direct contagion, the individual is directly contaminated by the source of infection, for example by sexual contact or by airborne transmission.
Indirect contagion, on the other hand, is mediated by animated vectors, such as animals and insects, or inanimate ones (air, soil, food, water, personal effects, toys, money, etc.); towels and linen for mixed use, as well as swimming pools and public baths, can be a source of indirect contagion for some diseases (usually mycosis).
Contagion mode
Contagion implies the "entry and possible multiplication in the organism of pathogens (which may have bacterial, viral, fungal or protozoal origin); this infection can lead to the latent or overt state of the disease; during its course, the infectious agents they can therefore escape from the organism and constitute a source of contagion for other individuals.
In the case of inter-human contagion (from man to man), the transmission of the disease can occur by direct contact with a sick person or with a healthy carrier (the one who carries the disease without showing symptoms). In other cases the infectious agent is not transmitted by man but by animals; in this case we speak of anthropozoonosis (examples are brucellosis, rabies, toxoplasmosis, leptospirosis and plague).
The main methods of contagion occur by:
- aerial
- fecal-oral
- parenteral
- transplacental
Contagion by Air
It occurs through droplets of saliva or sputum emitted through coughing, sneezing or even through speech (measles, mumps, flu, whooping cough, rubella, scarlet fever, tuberculosis, meningitis, pneumonia). Contagion is all the more probable the higher the number of individuals in a confined environment (communities, crowded places, etc.).
Contagion by the Sexual Way
It occurs through small lesions, secretions and organic fluids with which one comes into contact during sexual activity (venereal diseases, including AIDS, hepatitis B, trichomoniasis, syphilis, gonorrhea, genital warts, chlamydia and genital herpes).
Contagion for Via Orofecale
It occurs through the ingestion of germs acquired through the digestive tract, through contaminated water or food; it is a typical example of indirect contagion (hepatitis A, typhus, cholera, polio, salmonellosis, dysentery, ascaridiosis, tapeworm, amebiasis, giardiasis).
Contagion for Via Parenterale
It occurs through even minor skin lesions; this is the case, for example, of tetanus and anthrax, in which the contagion occurs indirectly through the penetration of bacterial spores present in the environment. Other times the contagion is caused by blood transfusions or by mixed use of syringes (hepatitis, HIV), animal bites (rabies) or insect bites (dengue, malaria, yellow fever).
Contagion through the Transplacental Way
The placenta allows the passage of antibodies by endocytosis but prevents that of many pathogens (exceptions, for example, rubella viruses, syphilis bacteria and toxoplasmosis protozoa).
Many diseases recognize different modes of contagion. The cold, for example, can be transmitted directly through sneezing, coughing and phonation, but also indirectly by shaking hands or manipulating contaminated objects (in viruses they can last about three hours in the external environment).
