How does the cold virus – as opposed to the immune response – actually operate?

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My understanding is that all the stuff that sucks about a cold – runny nose, cough, sore throat etc – comes not from the cold itself, but the body’s immune system working to kick it out. What would actually happen inside you if a cold was just allowed to get on with it without the immune system intervening? Is it deadly?

In: Biology

A virus reproduces by injecting itself into a cell, forcing that cell to make a shitton of copies of the virus, and then bursting the cell to release the copies. You wouldn’t last long at all without an immune system. My gut reaction is that *any* virus would be fatal in that case.

There are still people who have poorly functioning immune systems. We call them “immunocompromised” and it’s very dangerous if they get sick.

The cold virus attaches to the outside of the cells lining your nose, throat, and lungs. It then inserts itself into the cell and starts hijacking the cellular machinery to make more copies of itself. Its outer “shell” of proteins comes apart and it releases what’s called RNA – a complex molecule similar to DNA. The viral proteins cause the cell to stop making copies of its own RNA, and instead start making copies of the viral RNA.

Now, RNA is like a template for protein manufacture; rather large molecular machines in your cells called ribosomes take a strand of RNA and “translate” its code into proteins. The viral RNA instructs the ribosomes to make more viral proteins, which then hijack other cellular machinery to assemble the virus’s “shell.”

This would all be fine and dandy, except your cells need to be making their *own* proteins, not viral proteins. As more copies of the virus are created, more of the cell’s ribosomes are “used up” by the viral RNA instead of doing, y’know, things that are actually useful and necessary for the cell to live. If a cell can’t make its own proteins, it dies.

Fortunately, cells are able to sense when they’re not functioning properly, and release a cascade of signaling molecules that activate what can be thought of as a suicide response – if the cell can’t repair the damage being done, it destroys itself. This often releases the signaling molecules into the space around it, where they are sensed by other cells nearby. These cells then start destroying themselves as well, due to the presence of these signaling molecules.

Your immune system has several “layers” of defense to try and stop this cascade from destroying your whole body, but I feel like an explanation of this would be beyond my ELI5 capabilities.