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Flu Symptoms and Diagnosis

Learn the common symptoms of the flu and how it can be diagnosed. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Stanford School of Medicine.

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Video transcript

Female Voiceover: At this point in your life, you've probably been infected with the influenza virus. So think back, if you can, on what was probably a pretty miserable experience. Think of the symptoms that you had. All the symptoms that we feel when we're sick with the flu are related to the course that the virus takes as it moves through our body. But first the virus has to get into our body. Let's say that someone who is infected with the flu sneezes or coughs near you and they spray a bunch of tiny droplets into the air. Each of these droplets can contain millions of influenza viruses. And if we breathe this in through the nose or the mouth, we can get infected this way. Another way we can get infected is, let's say some of these droplets settle on a surface like a table for instance. The flu virus can survive on a surface like this for two to eight hours. So if we're unfortunate enough to touch a surface like this and then touch our mouth or our nose we can get infected this way as well. This is why practicing good hand hygiene is so critical in preventing the spread of disease. So don't forget to wash your hands, especially during flu season. Now in this case, unfortunately the virus has entered into our system. It's moving its way through our body. It's going to infect cells along the way and cause cell damage. The damage that these cells experience is going to show up as the symptoms that we're so familiar with. As the virus moves in through the nose, we might get some congestion. Then the virus is going to make its way down into the throat, giving us a sore throat. Then it will get all the way into the lungs causing a cough. In response to the damage that the virus causes the body, the immune system is going to increase the temperature of the body in hopes that this will make viral replication more difficult. This is why we get fever and chills. This battle that the body is fighting against, the influenza virus, consumes a lot of energy. This is why we are so exhausted when all of a sudden we're in bed for days. It's miserable. A lot of common viruses can cause these symptoms, right? The common cold, for instance, can give us a runny nose. So how do we know that it's actually the influenza virus specifically that is making us feel so sick? Typically a person infected with influenza virus will have a fever of over 100 degrees, along with either a sore throat or a cough. This combination of symptoms is what we call an influenza-like illness. We say influenza-like illness because it's not yet confirmed with a laboratory test. It's just a combination of symptoms. We shorten this by saying ILI. During flu season between December and February, doctors who see an influenza-like illness will usually attribute this to an infection with the influenza virus without getting a laboratory test. So next time your friend says, "I had the flu last week. I was completely congested." Now you know you can say, "Well did you have a fever with either a sore throat or a cough?" Because that is what you really need for the clinical diagnosis of an influenza-like illness. Some doctors actually might want to confirm their diagnosis of influenza with a lab test. They do this by using something that looks kind of like a giant Q-Tip. They go straight to the source. Let's think about the places that you'd most likely find an influenza virus in someone who is sick. You'd go to the nose and you'd go to the throat right? Remember this is why we have nasal congestion and a sore throat, because the virus is hanging out here. We call this a nasopharyngeal swab. Naso for nose and pharyngeal for throat. We're going to take this nasopharyngeal swab into the nose, into the back of the throat, to collect some of these cells. We're going to send this to the lab to find out if in fact it's an infection with influenza virus. There are other kinds of lab tests that can confirm the diagnosis of influenza infection but this is the most common and most likely what you'll see in a clinical setting.