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Viruses are not actually living, they’re just an infectious bag of genes; in other words, they are some kind of coat that surrounds infectious material. This can be DNA, which we’ve all heard of - we’ve all got that - or its genetic relative called RNA.
Viruses are also tiny: a flu virus is about 1/10,000'th of a millimetre across. That’s too small for them to have any of the machinery they need to make new viruses inside themselves so they need to hijack a cell to do that. There are viruses for plants, there are viruses for animals and there are even viruses for bacteria - bacteria can catch a cold too!
The flu virus has receptors, which are like viral Velcro, on the surface of the virus particle. They will lock onto a target cell using these chemical receptors on the surface, which docks onto the cell surface. They then go into the cell. Once they’re in the cell they use it like a factory: they take it over and make it produce thousands or in some cases millions of copies of new viruses, which come streaming out of the cell. They infect other cells to make more viruses, or they escape from the body and infect a new victim.
When they’re damaging cells - when they’re infecting cells - they can potentially kill them - that’s called lytic infection. When they kill a cell that has a consequence for us because if it’s a cell in your airway, for example, it might damage the mucosa (the lining of your airway). This means you get inflammation, a rather blocked-up, sniffy nose. Plus, because you’ve got damage to the lining of the nose, you might get a bacterial infection on top so they can cause secondary infections.
Please let me know if you need any clarification. I'm always happy to answer your questions.
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