 What does the clinical course of COVID-19 look like? It tends to start with fever and cough, but before then, the average incubation period, the time between the moment you get sufficiently exposed to the virus and the moment you start showing symptoms, seems to be about five days. So almost a week goes by when you're infected and potentially infectious before you even know it. About 98% of those who are gonna start showing symptoms do so by day 12, which explains why people are typically asked to self-quarantine for two weeks after a potential exposure. After infection, apparent viral shedding may continue for more than a month, with an average of 20 days, though it's not clear how contagious survivors are, if at all, during that extended time period. The most common symptoms are fever and cough, experienced by about 90% and 70% of patients respectively, based on an analysis of more than 50,000 COVID-19 patients. In terms of less common symptoms, only about 4 in 10 experience fatigue, 3 in 10 cough up phlegm, and 2 in 10 experience muscle aches. Only about 1 in 10 suffer from gastrointestinal symptoms, such as nausea, vomiting, or diarrhea, or common cold-type symptoms like runny or stuffy nose, headache, or a sore throat. This is consistent with the regional concentration of ACE2, the receptors the virus latches on to, in the lungs rather than the nose or throat, though in pangolins ACE2 is found on their flicking anteater tongues. Here's a chart that can help you differentiate between COVID-19, the common cold, the flu, or seasonal allergies. The only COVID-19 symptom found predictive of a more severe course was difficulty breathing, which resulted in more than six times the odds of eventually having to be admitted into the ICU. That's why that's such an important symptom to catch early in a red flag to immediately seek medical attention. If it's going to strike, shortness of breath usually hits a week after symptoms begin. The notion that the course of about 80% of cases are quote-unquote mild was derived from an analysis by the Chinese CDC that was based on nearly 45,000 confirmed cases, while there are certainly mild and even asymptomatic cases. It's important to understand what mild means to the Chinese CDC. Its definition of mild included those with so-called walking pneumonia, meaning pneumonia not dire enough to require supplemental oxygen or hospitalization, but pneumonia nonetheless. Certainly not the common cold type courses people might think of when they hear the word mild, though the cases were at least mild enough that people should be able to treat themselves at home. The remaining 20 or so percent of confirmed cases were classified as severe, about 15%, which involved difficulty getting enough oxygen, or critical, 5%, encompassing respiratory failure, septic shock, and multi-system organ failure. About half of those critical cases died. So about 80% mild, 15% severe, and 5% critical, though that doesn't include all the asymptomatic cases that escaped detection completely and never become confirmed cases at all. Because as many as four-fifths of cases are asymptomatic, the risk of dying after being infected may end up ranging from 1 in 1,000 to about 1 in 300, though if you do show symptoms, the risk of death may be more like 1 in 150. In autopsy, the respiratory surface of the lung under a microscope appears obliterated by scar tissue. Pulmonary fibrosis, lung scarring, is expected to become one of the long-term complications among survivors of serious COVID-19 infection. A six-month follow-up of SARS survivors found that about 1 in 3 showed evidence of scarring on x-ray end up to 1 in 6 suffered a significant impairment in lung function. Death from COVID-19 comes from progressive consolidation of the lung, meaning your lungs start filling up with something other than air. In the case of regular pneumonia, that's largely pus, in COVID-19 pneumonia, postmortem show you drown in lungs that are filled with clear liquid jelly. But the good news is that there are modifiable risk factors for death and disability. From COVID-19, meaning things you have control over that can reduce your risk, which we'll explore next.