What Is COVID-19? Symptoms, Causes, Diagnosis, Treatment, and Prevention


COVID-19 mainly spreads through respiratory fluids (droplets and small particles called aerosols that float in the air) emitted from an infected person’s mouth or nose when they cough, sneeze, speak, sing, or breathe, according to the WHO.

Another person can become infected if they inhale the fluids or if the fluids come into contact with their eyes, nose, or mouth.

The WHO says the risk of COVID-19 transmission is highest in three settings:

  • Crowded places
  • Close-contact settings, especially where people have conversations very near each other
  • Confined and enclosed places with poor ventilation

Anyone with COVID-19 can spread it, even if they don’t have symptoms, says the CDC.

Age and COVID-19 Risk

Older people (especially those over age 50) who contract COVID-19 are at higher risk for a more severe case than younger people, with the risk increasing with age, says the CDC. Most COVID-19-related deaths occur in people older than 65.

Seniors who are unvaccinated or who have a disability, weakened immune system, or certain medical conditions are at especially high risk.

Health Conditions and COVID-19 Risk

A number of chronic diseases are known to increase the chances a person will develop severe illness from COVID-19. According to the CDC, the conditions that demonstrably increase risk in adults of any age include:

Children at increased risk of severe COVID-19 include those with complex medical problems; neurological, genetic, or metabolic conditions; and congenital heart disease. Children with conditions such as obesity, asthma, or chronic lung disease, diabetes, or sickle cell disease, or who are immunocompromised, are also particularly vulnerable to severe COVID-19, per the CDC.

Variants and COVID-19 Risk

All viruses, including the virus that causes COVID-19, constantly change by mutating, creating new variants and subvariants. In many cases these new forms of the virus quickly disappear or remain at very low levels, but sometimes they spread, displacing their viral competitors.

SARS-CoV-2 variants have evolved over time, from alpha, beta, and delta to omicron, the currently dominant variant, says Yale Medicine. Subvariants of omicron are considered especially contagious because of the high number of mutations on their spike protein, the part of the virus that attaches to human cells.

The three COVID-19 vaccines available in the United States significantly decrease the risk of serious illness and death caused by omicron and its subvariants. When so-called breakthrough infections occur in people who have been immunized, those illnesses are likely to be less severe than they would be in unvaccinated individuals, says the Infectious Diseases Society of America (IDSA).



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