Everyone has had a cold and everyone will continue to get colds. Science, and your doctor, have tried to eradicate the common cold, but to no avail.
Why are we still saddled with the common cold. Lets go to the root cause, which is, excuse me, are viruses, and by viruses I mean uncountable millions. Common cold viruses are so numerous that no one has attempted to even count them, common cold viruses can literally appear and die off or mutate into a different strain without anyone knowing.
In the old days viruses where unique digital organisms that would appear and never change. After a while viruses would "mutate into sever distinct strains, as their original creator or another entity made changes.
Nowadays malware writers cross-pollinate between different malware code attempting to create the uber malware. Then there was the so called storm worm which spread through a barrage or constantly changing e-mails with different intriguing subject lines and different executables.
The bottom line is that malware is here to stay with us for the foreseeable future, and just like real life there are some times where we must take extra care to avoid infections.