Why do symptoms of viral illness come on so suddenly days after exposure?

Submitted by CoveredinCatHairs t3_xu646t in askscience

After a certain amount of time has passed after exposure, what triggers the body to abruptly start displaying symptoms of illness such as fever, body aches, headache, etc.?

(Curious because I felt completely fine all day at work on Thursday, and then within a 10-15 minute window my back and joints began to ache and I developed a blinding headache and exhaustion- tested positive for Covid less than an hour later.)

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Polar87 t1_iquxx5z wrote

Viruses grow just like bacteria at an exponential rate. But where they differ is that bacteria do not need hosts to replicate while viruses do. This means that viruses need to invade the cells and manipulate them in such a way that they get them to reproduce the virus. This process takes a comparatively long time and once the cell releases the viruses, it spreads a whole bunch of them at once. This can perhaps cause the evolution of viral particles to be bit more "burst" like where you have a big growth in a short time span.

https://www.nature.com/articles/s41598-021-02148-8

But I doubt that really explains it. Usually when you start feeling sick, such as getting the typical chills and body aches, you're feeling your body's reaction to the virus more so than the virus itself. How fast and how hard the body reacts to the infection is dependent on the kind of virus, so it not illnesses will present themselves as sudden.

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Connect_Eye_5470 t1_iqycmj2 wrote

The short answer? Viral load. The number of viruses you are initially exposed to matters a LOT to how quickly and how severely your symptoms occur. You just 'caught a whiff' and the viral colony started small. Then it started to breed. Reached 'virulency threshhold' and voila you're 'suddenly' sick as a dog.

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aTacoParty t1_ir2nyhg wrote

Virus, particularly respiratory viruses, can inhibit your immune system to evade detection. This along with low viral titers contributes to the incubation phase of a viral infection (pre-symptomatic). The viruses continues to replicate unhindered until it can no longer suppress your immune system usually due to accumulation of viral particles, host cell death, or deleterious mutations in the viral genetic material during rapid replication. The release of inhibition is when you begin to feel sick as your body quickly ramps up your immune response. You can think of this like someone holding a door shut while you try to open it and they suddenly walk away.

Another factor is that your immune system can increase activation exponentially at first. Every tissue in your body has resident immune cells that can first sound the alarm. This will trigger a flood of new immune cells which then see an active infection and also signal for more help (via cytokines). Sometimes this response can be so intense that it can be life threatening which is in part why some very severe COVID cases are treated with steroids which actually inhibit the immune system.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3185581

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CoveredinCatHairs OP t1_ir2ogjv wrote

By intense immune response, do you mean a cytokine storm/multi system inflammatory syndrome?

This was a really helpful answer, by the way. Thank you.

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aTacoParty t1_ir2r5sa wrote

Yes! But the response doesn't necessarily have to reach the level of cytokine storm for anti-inflammatories to be helpful.

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Aarush0110 t1_iqum4uj wrote

Viral infections are usually related to the virus first invading the cell and then replicating there. This whole thing takes time as the virus first needs to pass through the physical barriers and then replicate and produce toxins which cause the symptoms.

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Rather_Dashing t1_iqv2w56 wrote

Doesn't sound right, do you have a source? Most early symptoms (especially those OP mentioned) are related to the immune response and the immune response could be triggered before the virus even enters cells, or more likely sometime after when the virus has replicated to high enough levels to trigger a sufficient response. In any case, it certainly should take days for the first viral particle to enter and replicate for the very first time.

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Ihavepurpleshoes t1_iqvdoel wrote

Do you have a reference for your assertion that “viruses produce toxins”?

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