Viruses are sort of diabolical--"sort of" because, while they are experts in causing shenanigans, they aren't doing it on purpose. Plenty of organisms get by in some pretty gruesome ways, but that doesn't make them evil. Viruses are roughly equivalent in malice to orca whales,
who subside solely on cute and cuddlies like seals, penguins, and baby grey whales...
Yes, it definitely doesn't make them worthy of a children's movie (oh, right... Free Willy--nevermind), but they're not evil. It's the nature of life: for one organism to survive, thousands of others must die. But okay, enough philosophy and orca-bashing.
Here is what viruses actually do when they make you feel like poop:
Once inside your body, the virus browses for a cell it wants to infect. Viruses are very picky; they need to find a cell that will match its surface proteins. This is why viruses specialize in one type of cell or tissue.
Once the virus finds a match, it attaches itself to the outside of the cell and injects it with viral DNA, also known as the seed of evil.
It's like Goldilocks, except instead of eating porridge and test driving some mattresses, she cleans out the fridge and sets the living room aflame.
Now all these new viruses are being crammed into your cell, which is already brimming with cellular necessities like organelles, cytoplasm, and the will to live. So now we're fresh out of room. The viruses become so numerous and bulky, they cause the cell to burst.
The cell is now dead and hundreds of baby viruses scatter in all directions to repeat the process on new, unsuspecting cells. Not. Cool.
But don't worry! The aforementioned series events only happens if the virus is feeling productive. If a virus is feeling lazy, it (like me) procrastinates. It may enter your cell and just hang out for days, weeks, months, or even years. But whenever it wakes up from its stupor, it will make viruses and destroy your cell... so I suppose you should worry. Sorry.