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Coronavirus disease 2019 (COVID-19) is a contagious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first case was identified in Wuhan, China, in December 2019. It has since spread worldwide, leading to an ongoing pandemic.

Symptoms of COVID-19 are variable, but often include fever, cough, fatigue, breathing difficulties, and loss of smell and taste. Symptoms begin one to fourteen days after exposure to the virus. Around one in five infected individuals does not develop any symptoms.[9] While most people have mild symptoms, some people develop acute respiratory distress syndrome (ARDS). ARDS can be precipitated by cytokine storms,[10] multi-organ failure, septic shock, and blood clots. Longer-term damage to organs (in particular, the lungs and heart) has been observed. There is concern about a significant number of patients who have recovered from the acute phase of the disease but continue to experience a range of effects—known as long COVID—for months afterwards. These effects include severe fatigue, memory loss and other cognitive issues, low-grade fever, muscle weakness, and breathlessness.[11][12][13][14]

The virus that causes COVID-19 spreads mainly when an infected person is in close contact[a] with another person.[18][19] Small droplets and aerosols containing the virus can spread from an infected person's nose and mouth as he breathes, coughs, sneezes, sings, or speaks. Other people are infected if the virus gets into their mouth, nose or eyes. The virus may also spread via fomites (contaminated surfaces) although this is not thought to be the main route of transmission.[19] The exact route of transmission is rarely proven conclusively,[20] but infection mainly happens when people are near each other for long enough. It can spread as early as two days before infected persons show symptoms (presymptomatic), and from asymptomatic individuals. People remain infectious for up to ten days in moderate cases, and two weeks in severe cases. Various testing methods have been developed to diagnose the disease. The standard diagnosis method is by real-time reverse transcription polymerase chain reaction (rRT-PCR) from a nasopharyngeal swab.

Preventive measures include social distancing, quarantining, ventilation of indoor spaces, covering coughs and sneezes, hand washing, and keeping unwashed hands away from the face. The use of face masks or coverings has been recommended in public settings to minimise the risk of transmissions. There are several vaccine candidates in development, although none has completed clinical trials.

Although work is underway to develop drugs that inhibit the virus, the primary treatment is currently symptomatic. Management involves the treatment of symptoms, supportive care, isolation, and experimental measures.

Signs and symptoms Main article: Symptoms of COVID-19

Symptoms of COVID-19[21] Symptoms of COVID-19 are variable, ranging from mild symptoms to severe illness.[22][23] Common symptoms include fever, cough, fatigue, breathing difficulties, and loss of smell and taste. People with the same infection may have different symptoms, and their symptoms may change over time. For example, one person may have a high fever, a cough, and fatigue, and another person may have a low fever at the start of the disease and develop difficulty breathing a week later. However, in people without prior ears, nose, and throat (ENT) disorders, loss of taste combined with loss of smell is associated with COVID-19 with a specificity of 95%.[24]

As is common with infections, there is a delay, known as the incubation period, between the moment a person first becomes infected and the appearance of the first symptoms. The median incubation period for COVID-19 is four to five days.[25] Most symptomatic people experience symptoms within two to seven days after exposure, and almost all symptomatic people will experience one or more symptoms before day twelve.[25][26]

Around one in five people are infected with the virus but do not develop noticeable symptoms at any point in time.[27][28][29] These asymptomatic carriers tend not to get tested, and they can spread the disease.[30][31][29] Other infected people will develop symptoms later (called pre-symptomatic) or have very mild symptoms, and can also spread the virus.[32] Cause COVID-19 is caused by infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus strain.

Transmission Main article: Transmission of COVID-19 COVID-19 spreads from person to person mainly through the respiratory route after an infected person coughs, sneezes, sings, talks or breathes. A new infection occurs when virus-containing particles exhaled by an infected person, either respiratory droplets or aerosols, get into the mouth, nose, or eyes of other people who are in close contact with the infected person.[33][34] During human-to-human transmission, an average 1000 infectious SARS-CoV-2 virions are thought to initiate a new infection.

The closer people interact, and the longer they interact, the more likely they are to transmit COVID-19. Closer distances can involve larger droplets (which fall to the ground) and aerosols, whereas longer distances only involve aerosols. The larger droplets may also evaporate into the aerosols (known as droplet nuclei). The relative importance of the larger droplets and the aerosols is not clear as of November 2020, however the virus is not known to transmit between rooms over long distances such as through air ducts. Airborne transmission is able to particularly occur indoors, in high risk locations, such as in restaurants, choirs, gyms, nightclubs, offices, and religious venues, often when they are crowded or less ventilated. It also occurs in healthcare settings, often when aerosol-generating medical procedures are performed on COVID-19 patients.

Social distancing and the wearing of cloth face masks, surgical masks, respirators, or other face coverings are controls for droplet transmission. Transmission may be decreased indoors with well maintained heating and ventilation systems to maintain good air circulation and increase the use of outdoor air.[34]

The number of people generally infected by one infected person varies; as of September 2020 it was estimated that one infected person will, on average, infect between two and three other people.[35] This is more infectious than influenza, but less so than measles.[36] It often spreads in clusters, where infections can be traced back to an index case or geographical location. There is a major role of "super-spreading events", where many people are infected by one person. Virology