Welcome to the first lecture in the Stanford COVID-19 lecture series. I am Dr. David Hao, here with Dr. Mahadevan, and we will be giving you an introduction to COVID-19 based on the currently available data. We will review key signs and symptoms, discuss the spectrum of illness, and walk through the clinical course. After this lecture, you will learn how COVID-19 is transmitted, including asymptomatic spread and multiplying growth, recognize at risk populations, identify key signs and symptoms, and you will also be able to define the spectrum of illness, which can range from an asymptomatic patient to a very severe illness, describe the typical course of illness, and understand that patients may have atypical presentations. COVID-19 is likely spread via coughing, sneezing, or even talking and breathing, producing respiratory droplets that typically travel less than two meters and can remain in the air or land on surfaces. Individuals can acquire the infection by inhaling the viral particles or touching contaminated surfaces. Be aware that the disease can be spread even before symptoms appear. Infectious particles which are invisible to the naked eye can be on any surface, such as door handles, sides of gurneys, keyboards, and floors. And remember that the infection can spread after contaminating your hands and then touching your face. Currently, one person infects about two to three people. This is the R naught or basic reproduction number of the virus. What this means is that one person can potentially trigger an infection of over 6 thousand people within weeks. This R naught is significantly larger than seasonal influenza, which means that COVID-19 is more easily transmitted from person to person. An important point is that the R naught can be reduced with public health interventions such as physical distancing and hand-washing. Here in South Korea, a woman, patient 31, went to a church with contacts of over 1,000 people. Incubation can take up to 14 days before symptom onset. Most patients develop symptoms around four days after exposure. This is different than seasonal influenza, where people can suddenly develop symptoms within hours of exposure. However, patients can be infectious before they develop symptoms, or they may never develop symptoms. This may lead to significant silence spread of the disease. The mortality rate, or as it is also known, the case fatality rate (CFR), for COVID-19 infections varies from 0.7% to 7.2%. The true case fatality rate will remain unknown until we were able to do more widespread testing because it really depends on the total number of cases that are detected. Older patients are at most risk for complications and death. Other risk factors for more serious disease and complications include high blood pressure, diabetes, cancer, coronary heart disease, and chronic lung and chronic kidney disease. Not all patients with COVID-19 will develop symptoms. In Iceland, a large population was tested. 50% of people that tested positive had no symptoms. And not all patients will develop a severe illness. Many will recover without knowing that they had an infection, while others, especially those with risk factors, will be at significant risk for severe illness. About 81% of patients will develop a mild infection around day four. These patients may have an influenza-like illness with fever, cough, and sore muscles. They also may have loss of taste and/or smell. However, about 14% of all cases go on to develop a severe illness around day nine. These patients will complain of shortness of breath. They typically have a severe pneumonia with greater than 50% of their lung involved, and as a result, they suffer from hypoxia, which is a lack of oxygen to their vital tissues, and approximately 5% of all patients will become critically ill around day 12. These patients will develop respiratory failure and often require ventilatory support. Some will develop shock and multiorgan dysfunction. Some will die, while others may recover. But there are currently no validated scoring systems to identify who are at highest risk for death. The average time for recovery for mild cases is about two weeks, and for severe cases, it may take three to six weeks. And now our colleague, Dr. Betty Chang, wants to share an important lesson from her experience on the front lines of the COVID outbreak in New York City. You know, having a suspicion and keeping our guards up has been what we sort of incorporate in our daily practice just because of the different type of symptoms it presents itself. We know that patient who tested positive will remain positive for weeks even if their symptoms go away. So, even if we are plateauing, I think it is important for us and that's what we've been practicing to keep our guards up.