Since viruses do not have many genes, they can easily mutate into something new. That's one of the reasons there isn't a common cold vaccine or an effective flu vaccine. This year's flu vaccine was less that 40% effective and some years it is less than 10%.
With the advent of the electron microscope in 1931, we got our first pictures of a virus. They are very tiny and are much smaller than bacteria. Viruses range from 20 to 300 nanometers (nm--billionths of a meter) while bacteria are at least 1000 nm--also very tiny! But this is why viruses can infect bacteria. Since the human body has over 100 trillion bacteria, you can sense the problem. You are made up of more bacteria than human cells except the cells are larger! Scientists don't know whether to categorize viruses as living or dead. They cannot do anything on their own. They require a living entity or host to infect and replicate (make copies of themselves). Unlike humans that have 20,000 - 25,000 genes, viruses can reproduce rapidly because they only have 8 to 11 genes.
Like a Trojan Horse, once the virus gets inside the cell, it unleashes its own genetic material (DNA/RNA) and convinces the cell manufacturing plants to make copies of itself rather than what the cell normally does. The new baby viruses created within the cell are budded (birthed) out of the cell and travel to other neighboring cells to continue their work of infection and regeneration. The only way to get rid of them is for the cell to self-destruct (apoptosis) or signal the need to be destroyed by a white blood cell warrior (macrophage) working for the immune system.
The good news is if we can keep the virus from attaching to a cell, then it cannot infect the cell. Specific sugars (glycans) on the outside of the cell, and on the virus as well, are what enable the attachment to occur. Like a key and lock, only a certain key can turn the lock and open the door to the cell. This is why a virus can only gain entry to a limited number of cells. The correct receptor must be on the surface of the cell. Healthy human cells often have long, cell surface sugar structures, that act as decoys and deflector shields. These large antennas are like trees in a forest and can block a virus from getting down to the shorter shrubs (more vulnerable cell receptors) near the surface of the cell. If the glycocalyx (shroud of sugars around the cell) is not healthy with lots of sugars, then your cells can be more easily compromised. Maintaining healthy cell receptors is vital to staying well and defending against a viral threat.
For more information on viruses and glycans, see the book, There's An Elephant in the Room--Exposing Hiddent Truths in the Science of Health.