There are a couple of issues (or should that be tissues) going on here.
Firstly, tim is right in highlighting the over and mis-use of the word "flu" which specifically refers to infection by the influenza virus. This is a nasty virus that kills thousands of people annually all around the world. The excess mortality in Australia where Tim lives is a couple of thousand people annually, for instance.
But 'flu hits hardest those that are weakest. We tend to have the worst doses when we are young and old. The young have no pre-existing immunity to the virus and hence the first time they catch it the virus has a head start and makes them much more unwell. The elderly have co-existing illnesses and a weaker immune system and are hence more susceptible to its effects.
Common colds, on the other hand, produce more mild infections, largely because we have some underlying immunity against them.
Vaccination against the flu can provide protection and lessen the severity of infection because it gives the immune system a headstart over the pathogen, preventing the infection from getting going properly.
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx