Why are we no longer contagious even if not all influenza symptoms are gone?

Submitted by cazzipropri t3_z35yvm in askscience

We are told that influenza type A is likely no longer contagious 48 hours after fever goes away, but other symptoms (such as cough, sneeze, mucus) may linger around long after that.

It seems natural to me that cough, sneeze and mucus are all vehicles for the virus to spread, and it seems strange to me that the lack of fever should have much correlation with how infections these vehicles remain.

What are the influenza contagion mechanisms that determine when you are contagious?

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sciguy52 t1_ixl0mpl wrote

So couple things. The symptoms you get from the flu are not from the virus itself, it is the immune response to the virus. Don't believe that? Well when you get infected with the flu (or cold), you are infected and infectious before you have any symptoms at all. So there is a day, maybe two you are spreading it around but are not aware you are even infected. My point with this is to show you it is not the virus infecting you causing the symptoms otherwise you would get them as soon as you were infected. Then you start to get symptoms. As mentioned those symptoms are a function of the immune system fighting the virus, not from the virus itself. When you get infected with a virus interferon will be released in your body to help stop the spread of the virus internally. Also so happens interferon makes you feel flu like, and that is likely part of the reason you feel sick (although I am sure some other aspects of the immune system contribute to. But if you ever head to inject interferon as MS patients do, you would notice you feel sick with flu like symptoms. OK we have established your symptoms are related to your immune response, not from the virus itself. So you have been asymptomatic then sick for about 6 days and you are getting a little better but still have symptoms but don't seem infectious anymore. What gives? Well during that time you body has been working hard to make antibodies against the virus as well as cytotoxic T lymphocytes and other to destroy not only the virus, but also the virus infected cells. Once you body has produced antibodies against the virus they work surprisingly quick. Those antibodies will bind to any free virus which results in other cells swallowing and destroying the virus in some instances, or the antibodies might bind to the virus in a way so that it cannot infect another cell as the antibodies block the viral binding protein. When you reach this point you are not going to be infectious as there will not be free virus available to cough or sneeze out. The antibodies are very very efficient at finding their targets fast, and the the rest of the immune response to that antibody bound virus is fast too. At this point you are not contagious but you immune system is still involved in the fight so you will retain some symptoms for a few days that slowly get better. During that later period your cytotoxic T lymphocytes found and killed every cell that had the virus in it, all of them. The antibodies clearered the virus outside of cells including in nasal secretions etc. You then gradually feel recovered and this is when the immune system has not only won the battle, now it is time to throttle back the immune response as there is no more virus present. As it does this your symptoms disappear as your body goes back to "healthy mode" where the immune activity does not make you feel ill. This is simplified a bit as the immune system is quite complex but captures it pretty good.

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RebelWithoutAClue t1_ixl3ctp wrote

It is interesting that so many differently named viral infections have such common symptom sets. There may be a few things like blisters on the tongue and sores on the feet, but fever, chills, coughing, headache, are super common symptoms.

Not being able to see things directly it would be reasonable to conclude that much of our observable symptoms are more of a feature of how our immune system decides to mobilize than it is about a particular viral infection.

If this is true, our system warning light may also be turning off well after the situation has actually cleared.

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ZoomerBoomer42 t1_ixmpuhq wrote

I think it's more so that different viral infections lead to similar immune responses that then lead to common symptoms.

The coughing, sneezing, and runny nose are all symptoms of your body trying to expel irritants from your body. In the case of the flu, it's likely the inflammation from the immune response that causes these symptoms.

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sciguy52 t1_ixo2ebd wrote

You are correct. Many (but not all) viruses stimulate a similar immune response as with the flu. Then you feel "flu like". In truth, not all flu's are caused by the flu virus. There are other viruses that can create similar symptoms to the flu. Most of the time we just don't go and check every patient, and it resolves much as the flu does. But the immunological response is similar causing similar symptoms. Same with colds by the way. Most people think there is a "cold virus". Actually colds are caused by multiple different viruses. Rhinoviruses may cause 50% or so, but what most don't realize is maybe 25% are caused by corona viruses. Yes, same family as SARS-CoV2 but a bit more distantly related at a genetic level. But the symptoms are that of a common cold. The common cold has the added symptoms of a runny nose, which again is a function of the immune response to the virus, not caused by the virus itself.

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goofbeast t1_ixo9z84 wrote

Other cytokines, like TNF-alpha and interleukins contribute to these symptoms too. Inflammatory substances like prostaglandins are generated as part of the inflammatory reaction, and these activate pain nerve endings. Generation of these inflammatory substances in the infected airways causes sore throat and also trigger nerve endings wich activate cough and sneeze reflexes.

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Tym4x t1_ixm2tu6 wrote

The symptoms are a result of the immune system reacting to it. Even when the culprit is gone, it can take a while for affected areas to calm down & renew broken tissue, which is also why many people still cough for weeks after an infection.

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baldeagleNL t1_ixnkmkp wrote

It's very easy to confuse 'being ill' with 'being infectious'. Those are only loosely related. You're generally infectious when the amount of virus particles in your airways is the highest. But influenza symptoms can exist long after you have cleared the virus from your system. The damage the viruses did can linger for days to weeks to years. The post-viral syndrome (now often referred to in the context of COVID-19 as post-COVID) is a real issue for a lot of people.

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