Tsaeb,
Many colds look and behave the exact same way because all they sometimes are, are variants (mutations) of similar virus'. Bacteria and virus' change and alter all the time. They adapt to new hostile (read "Resistant") envirnoments. These organisms have as much vested interest in surviving as WE do, so THEY change and adapt like WE do.
Humans learn to do new things to survive, these organisms alter themselves to accomplish the same thing.
In the end, though, what this means is that these variands make the body react the same way, make the same illness-appearing symptoms and we feel the same (crappy) way while fighting off the infection.
The only way to truely know which the 'bug' is would be to plate, culture and examine through a microscope WHICH infectious organism it is.
WIth bacteria, antibiotics are used to fight off pernicious infections that do not go away with normal body-defenses and time. With virus infections, we can only wait them out for the most part.
Also, another thing to realize is this. Your body gets worn down in its defenses. It needs time to regroup, rebuild and get strong again. Ok, you DID fight off an infection. Well, ANOTHER one decided that with your low resistance and resouces that this was the IDEAL time to move in and strike while the iron was hot. AND it was RIGHT!
Think of it like this. In war-fare in the centuries gone by, two countries/castles/whatever would fight until one of them won. Then when they BOTH were worn down, bloodied and beaten, ANOTHER competitor would move in and take over the spoils.
This is often what happens when you get sick. Many times there are two or three OR more different infections in a row that testing the defenses and trying to overwhelm the battlements.
THAT is why it is so important to take care of yourself, eat well, rest and give your body the BEST chance at a fast recovery.

I hope that helps.
End of lecture.
