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Coronavirus disease 2019 (COVID-19)

COVID-19 is a respiratory disease caused by the SARS-CoV-2 virus. Since the virus was identified in 2019, it has spread and created a global pandemic. Vaccines are effective at reducing both the chance of contracting COVID-19 and its severity.

Our evidence-based analysis on coronavirus disease 2019 (covid-19) features 47 unique references to scientific papers.

Research analysis led by and reviewed by Rob Stenstrom, MD, PhD and the Examine team.
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Summary of Coronavirus disease 2019 (COVID-19)


COVID-19 is a respiratory disease caused by the SARS-CoV-2 virus. Since it was first described in December 2019, the disease has spread globally. The SARS-CoV-2 virus is primarily spread through respiratory droplets, so risk is highest when people spend extended periods in close proximity with someone who is infected. There are many symptoms that COVID-19 and the flu have in common, but certain symptoms, such as a loss of smell or taste, can be a distinguishing factor.

Currently, the best way to avoid contracting COVID-19 is to get vaccinated, wear a mask, and practice social distancing and good hygiene habits. Activities that generally support immune health, such as being physically active, maintaining a healthy body weight, sleeping well, and following a good nutrition and supplementation regimen, are also likely to help.

Some important virus-related concepts

1. COVID-19 vs. SARS-CoV-2

COVID-19 (coronavirus disease 2019) is a disease, not the virus that causes it. The virus itself is called severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)”.

  • COVID-19 is the disease caused by SARS-CoV-2.

  • SARS-CoV-2 is the virus that causes COVID-19.

The virus owes its name to its genetic similarity to the original SARS virus, SARS-CoV, which was responsible for an outbreak of a similar respiratory disease in 2003.[1]

2. COVID-19

There are seven diseases caused by coronaviruses in humans; the most severe are Middle East Respiratory Syndrome (MERS), SARS, and COVID-19, caused by the viruses MERS-CoV, SARS-CoV, and SARS-CoV-2, respectively.[2] The other four (called HCoV-229E, HCoV-HKU1, HCoV-NL63, and HCoV-OC43) have long been widespread globally and cause 10–30% of all upper respiratory tract infections in adults.[3]

3. Incubation period

The “incubation period” of a virus is the time between a person first being infected and when they start developing signs and symptoms of the disease.[4]

The incubation period of SARS-CoV-2 has been reported as 2–14 days, with a median of 5–6 days. One case study suggests that it could be as long as 24 days.[5][6] That means you can be infected for quite a while and not feel sick.

4. Viral load

If you have a harmful virus in your body, less is better. The viral load is basically the amount of virus per volume of fluid (such as blood or sputum, the phlegmy stuff at the back of your throat). For COVID-19, the viral load appears to peak 5–6 days after symptoms start, but even people without symptoms can have a significant viral load, which suggests that the virus can be spread by a person before they become symptomatic.[7] 

The mean viral load of severe cases was around 60 times higher than that of mild cases, suggesting an association between viral load and symptom severity.[8] You can reduce your viral load by minimizing your exposure to the virus, through measures that include avoiding sick people when possible, frequently washing your hands for at least 20 seconds, and avoiding touching your mouth, nose, and eyes. Since the virus is spread primarily through the air, wearing a mask is the best way to limit exposure for other people and yourself.

Research on COVID-19

Cause & Transmission

COVID-19 is caused by infection with the SARS-CoV-2 virus.[9]

A number of transmission routes have been investigated, but it seems that SARS-CoV-2 is primarily transmitted between people within 6 feet of each other, through the air via respiratory droplets and aerosolized particles.[10] It’s possible that the virus can be transmitted over longer distances, but it’s not clear how much this has contributed to overall transmission.[11]

Similarly, although SARS-CoV-2 can technically remain on surfaces for extended periods of time[12][13] and are found in bodily fluids and secretions, it’s likely that direct contact with infected surfaces is not a major source of transmission.[14]

Different strains (i.e., versions with different genetic sequences and sometimes structures) can vary in terms of how easily they spread. For example, the Delta variant of SARS-CoV-2 (one of the dominant strains of infection in most countries, including the United States) may be more than twice as infectious as the original strain — but not more lethal.[15] Regardless of strain, a person’s physical proximity and duration of time spent around someone with COVID-19 predict their risk of contracting the virus themselves; the risk is higher the longer you spend with someone who is infected.[16][17][18][19]

COVID-19 is primarily spread through respiratory droplets. The risk of being infected is highest if someone spends a long period of time in close proximity to an infected person.

Treatment & Prevention


A number of vaccines have been approved for COVID-19, and more are being developed.[20]

Vaccines have been proven to reduce the risk of contracting COVID-19 in clinical trials, which has been supported by observational research conducted during a number of nations’ vaccine rollouts. Additionally, should vaccinated individuals still contract COVID-19, their risk of severe illness or death is dramatically lower than that of unvaccinated people.[21] It’s possible the available vaccines may be less effective against different strains of SARS-CoV-2 (most notably the Delta and Mu variants). There have been some noteworthy instances of large groups of vaccinated people contracting the Delta variant. Although concerning, these “breakthrough” cases remain rare — and severe COVID extremely rare — in vaccinated individuals.[22]

Vaccines are effective at reducing both the risk of getting COVID-19 and developing severe disease should you become infected anyway. (Breakthrough cases can happen, but they are rare.)


There is strong evidence that masks help prevent COVID-19 infections by reducing the spread of respiratory droplets while breathing, speaking, coughing, etc.[23][24][25][26][27]

There are many types of masks, including N95s, surgical masks, and cloth masks. As long as they fit correctly, N95s offer the best protection, and now that availability has increased, the CDC encourages their use in the general population.[28] In a pinch, though, a cloth mask is still a fine option and better than no mask at all.Cloth masks are also a good option for the general public because they are cheap and easy to clean. (Simply throw them in the wash.)

Other countries make masks that are roughly equivalent to American N95s but have different names, such as FFP2 (European Union), KN95 (China), and KF94 (Korea). All of these are appropriate options.

Tips for using a mask properly

A good mask will:[23]

  • Have 2 or more layers of breathable fabric

  • Completely cover your nose and mouth

  • Fit snugly on your face

  • Have a wire to help it fit around your nose

Avoid masks that:

  • Are made of non-breathable materials (like plastic)

  • Have a vent, which allows the virus to escape when you exhale

  • Should be saved for people who have high degrees of exposure (especially N95s for healthcare workers)

Masks can reduce COVID-19 transmission by blocking respiratory droplets. There are many types of masks, but for the general public, cloth masks are perfectly fine.

Social Distancing

Given that the risk of being infected with COVID-19 is highest when people are in close proximity for long periods of time, avoiding this (when possible) is an effective way to manage the spread of the disease.[29][30]


Hand-washing Good hygiene practices are a simple way to reduce the risk of disease in general and COVID-19 specifically. Coronaviruses are “enveloped viruses”, which means they have a lipid coating with spikes that allow them to bind more easily to targeted tissues. This coating makes coronaviruses more susceptible to soap and disinfectants, including alcohol-based wipes and hand sanitizers.

It’s important to wash your hands correctly. One study in a college town suggested that only around 5% of restroom users washed their hands long enough to remove infection-causing germs,[31] despite how essential this is for reducing virus transmission.[32][33][34][35][36] Also, many people don’t completely dry their hands, and wet hands increase transmission risk.[37]

To properly clean your hands, wash your hands for at least 20 seconds and dry them fully

Hand sanitizers Although hand sanitizers can disrupt the lipid coating of coronaviruses, they aren’t as good as handwashing. The friction produced by rubbing your hands together combined with the water pouring over them helps handwashing remove viruses more effectively than hand sanitizer.[38][39]

Other Medications

There aren’t many medications that have been shown to be effective against COVID-19. There is some proof that monoclonal antibodies might be beneficial for people who receive them shortly after diagnosis, but more evidence is required.[40][41]

Signs & Symptoms

The symptoms of COVID-19 typically appear within 2–14 days of contracting the virus and can range from mild to severe. They include:[42]

  • Fever or chills

  • Cough

  • Shortness of breath

  • Fatigue

  • Muscle weakness

  • Loss of smell or taste

  • Headache

  • Sore throat

  • Congestion or runny nose

  • Nausea or vomiting

  • Diarrhea


Among people who recover from COVID-19, most see their symptoms fully resolve, but some experience chronic symptoms. If symptoms last 4 weeks or longer, a person is said to have “long COVID” or “post–COVID-19 syndrome”.[43]

People who are diagnosed with long COVID often report a continuation of the symptoms listed above. Additional symptoms they have reported include:

  • Difficulty thinking or concentrating (brain fog)

  • Heart palpitations

  • A “pins and needles” feeling

  • Sleep issues

  • Lightheadedness

  • Rash

  • Mood changes

  • Changes in menstrual cycle

This aspect of COVID-19 is still poorly understood. The risk factors aren’t well known, but it’s possible that being female, being older, having a high BMI, and experiencing more than five symptoms during the first week of infection contribute to developing long COVID. [44][45][46]

Risk Factors for Severe Disease

Severe disease or death is more common in people with one or more of these risk factors:[47]

  • Male sex

  • Older age

  • Hypertension

  • Diabetes

  • Obesity

  • Chronic Obstructive Pulmonary Disease (COPD)

  • Chronic liver and kidney diseases

  • Cancer and undergoing chemotherapy

  • Pregnancy

  • Immunodeficiency or taking immunosuppressive medications

Research on Supplements for COVID-19

The table below displays an analysis of human studies and indicates how supplements may affect COVID-19.

Vitamin D

Vitamin D has a wide range of effects on inflammation and the immune system, which may help improve symptoms of COVID-19.


Curcumin’s anti-inflammatory properties are well studied and could possibly improve disease severity of COVID-19.

Things to Note

Also Known As

COVID-19, 2019 novel coronavirus disease

Do Not Confuse With

corona, SARS, flu

Caution Notice

This page is not a replacement for information from health authorities! Evidence is constantly evolving. Please use health authorities such as the WHO and CDC as your primary sources of information — they have hundreds of researchers and public-health officials working specifically on reducing the spread and impact of COVID-19.

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