[MUSIC PLAYING] SPEAKER: Now, let's talk about the natural history of SARS-coronavirus-2. And by natural history, I mean, what happens to someone when they get a SARS-coronavirus-2 infection. So what does happen to someone after they're infected with SARS-coronavirus-2? Well, many people get an illness that we call COVID-19. COVID-19 is short for coronavirus disease that emerged in 2019, and for short we say, COVID-19. So not everyone who gets infected with SARS-coronavirus-2 gets sick. So not everyone develop signs and symptoms. However, if they do get sick, we're going to talk about what their signs and symptoms are. We would want to know how long they're sick. We'll talk about that as well. Importantly, we're going to talk about how long they can infect people and spread the virus, and where and how people shed the virus. So these are all the different components of what happens to someone when they're infected with SARS-coronavirus-2. Let's talk, next, about signs and symptoms of disease. So you may have heard these words before, but it's important to define them so the difference between signs and symptoms. Signs of disease are objective measurements to characterize an illness or disease that can be observed during a physical exam. So, for example, temperature is a sign of disease, because it can be measured by taking your temperature. Breathing faster than usual is also a sign, because someone can count the number of times that you're breathing per minute and determine whether it's faster than usual. Symptoms are not something that can be objectively measured, but how the patient says they're feeling. So, for example, fatigue is a symptom, because it's how someone is feeling. They're feeling very tired. Nausea is a symptom, because it can't be measured. It's how someone is feeling. A loss of taste or smell is also something that someone reports. It's something that they're observing themselves, not something that can be observed during a physical exam. Muscle ache is the same. Fever can be both a sign and a symptom, because it can be measured by someone else and you can also feel when you have a fever. So it's important to keep these general definitions in mind. Now, let's talk about the signs and symptoms of COVID-19. And this disease is unusual, because it can cause such a wide variety of signs and symptoms. It makes it difficult to classify. So as we just mentioned, some people have no symptoms at all. This is called being asymptomatic. So this means that someone can be infected with SARS-coronavirus-2, and they will never know. They feel just fine. They're perfectly healthy. Other people will have some disease, but it will be mild. They may not need to seek care at all. It may feel like just a head cold, or a very mild fever, or respiratory illness. Other people will have very serious disease. They'll feel very sick, and it can lead to death in some people who develop severe disease. So, again, it's a wide range of signs and symptoms with this infection. Among people who do develop signs and symptoms, there are some common signs and symptoms of COVID-19. Many people will develop a fever. Many people will feel very tired, or have what we call fatigue. People may have chills. That often goes along with the fever. Some people will experience muscle pain. Some will develop a cough. Many people think about COVID-19 as a respiratory disease, and think about coughing, but not all people with COVID-19 will have a cough. Many people who have COVID-19 say that they recently lost their ability to taste or smell. Some will have difficulty breathing and feel like they're short of breath. That's a more serious symptom of COVID-19. Some people with COVID-19 could also have a headache and a sore throat. It's important to keep in mind that people who have COVID-19 may have only one of these signs or symptoms. They may have just a few of these signs or symptoms. Some people may have all of these signs or symptoms. So there's not one right way to define COVID-19. But you should be aware of all the possible signs and symptoms that people may have. And many signs and symptoms of COVID-19 are what we call non-specific. It means they're common to other infections as well. So fever, cough, myalgia or muscle pain, are common to many different types of infections. So people with these signs and symptoms may have COVID-19, but it's hard to tell without a test. Some signs and symptoms are more specific for COVID-19. I mentioned one previously, and that's the recent loss of the ability to taste or smell. About 1/3 of patients report this loss of taste or smell, and this is something that's a bit more specific for COVID-19, because it's very rare in other types of infections. There are some signs and symptoms that mean the disease is becoming more severe or progressing, and these are important to know, because when people have these signs and symptoms, they may need to seek immediate care. The most common signs of progression of the infection are increased or more severe fevers, an increased difficulty breathing. Anyone who is experiencing these signs and symptoms should contact a doctor. Patients should immediately seek emergency care if they develop any of these warning signs or symptoms, including, bluish lips or face. That could mean that they're not getting enough oxygen. If they have an increased rate of breathing, which means they're breathing much faster than normal. If they're feeling shortness of breath, which means they feel like they're not getting enough air in, even though they're breathing quickly. If someone is experiencing chest pain when breathing, this is a serious symptom and they should seek immediate care. If someone is waking up from sleep feeling like they're short of breath, and they're not able to get enough air in, they should seek care right of way. Anyone who develops new confusion, so they're confused about events, or people, or what's happening, or they're difficult to wake up, should also seek immediate emergency care. All of these are signs that the patient is not getting enough oxygen, and that's a very serious condition. And they should seek care right away. One of the ways that COVID-19 causes severe disease and death, is through congesting in the lungs. So our lungs are how we get oxygen into our body. And if the lungs don't function properly, we can't get enough oxygen. So on the left side here, you can see healthy lungs and what healthy lung tissue looks like. You can see here that the field is dark. This shows you that the tissue is healthy. On the right side, however, are the lungs of someone who has severe COVID-19. So in red here, you can see the same lung area that's pictured on the left, except you can see that it's white instead of dark. That shows that the tissue is severely damaged by the virus, and this shows that this person is having a very difficult time breathing and getting enough oxygen into their body. This shows you what the virus can do, and how it causes severe disease and death. Now, let's talk about the incubation period for SARS-coronavirus-2. It's important for you to learn the term incubation period, because you're going to hear it throughout this course. So the incubation period is the time from when someone is infected until they develop symptoms. So usually this takes a number of days. So the incubation period for SARS-coronavirus ranges from 2 to 14 days. So most people infected with this virus will develop signs and symptoms between 2 to 14 days after they're infected. You can see here on your screen the proportion of people who develop symptoms by each day after they are infected. So 5% of all people infected with this virus will develop signs or symptoms within two days. So it can be very quick. And 1/2 of people infected, will develop signs and symptoms within five days. So just five days after infection, 1/2 of everyone infected will develop signs and symptoms. By 14 days, 95% of all people who are infected will have developed signs or symptoms. OK, now let's define the infectious period. This is an important term that you'll need to know for the rest of the course. The infectious period is the time at which someone infected with SARS-CoV-2 can transmit the virus to other people. So for people who have COVID-19 disease, their infectious period starts two days before the start of their signs and symptoms. And they could infect other people throughout the end of their illness. So we define the end of their infectious period as being at least 10 days after the onset of their illness. And as long as their symptoms are improving, we could assume that they're no longer infectious. So it's a bit tricky to identify exactly when the end of the infectious period is, but these are some of the markers we use to identify when someone may no longer be infectious. As you know, some people who are infected with SARS-CoV-2 may never develop any kind of symptoms. They are asymptomatic. But they can still be infectious. They could still infect someone else. For these people, it's more difficult to define their infectious period. It's more difficult to define when they could infect someone else. But, typically, we assume this period to be about 10 to 14 days, and we should assume that it started before they had a test that showed that they had the infection, and to last for at least 10 days after the test. So now, let's think about the timeline of infection using these new terms that we've learned, the incubation period and the period of infectiousness. So here, first, we see that someone is infected by someone else. And then, they enter their incubation period. So the person is infected, but they haven't developed signs or symptoms yet. They may not know that they're infected. Remember, this incubation period can range from 2 to 14 days, but on average is about five days. The incubation period here is represented in the orange line. At the end of their incubation period, again, which will be an average of five days, but can be up to 14 days, they will develop signs and symptoms of disease. So for people who have mild illness, that illness is going to last about seven days. For people who have more severe illness, that can last two weeks or more. Now, let's think about when that person is infectious, or when could they possibly transmit the virus to someone else. So you can see here in yellow, that this person who developed signs and symptoms here on day 14 was infectious starting from day 12, and they are going to be most infectious on the first day of onset of symptoms, which will be around day 14. So on the day that they first notice that they are sick, the day they first start feeling badly, is the day that they are most infectious, or probably most likely to infect someone else. They can continue to be infectious throughout their signs and symptoms. This will be at least ten days, possibly longer. Particularly for people who have severe disease, they're going to be infectious for much longer and through the duration of their illness. So it's important to remember here a few key things. People can infect others before they become sick, before they know they're sick, and particularly on the day that they're sick. And they can continue to be infectious and transmit to others through out their illness period, as long as that illness period lasts. So that's going to vary from person to person, but we'll be at a very minimum for seven days. [MUSIC PLAYING]
