Question

If our cells develop imunity against a disease once we have it once(like smallpox), then why do we get common cold ?

Answer 1

The human body has excellent resistance to cold viruses. When was the last time you heard of someone dying from a cold? The reason colds are considered the archetype of "minor illness" is that we're resistant to them.

the reason we get repeated colds is that we're infected with many different viruses, all of which cause the same symptoms. Even though we do develop immunity against each one, we're then infected by something different.

(It shouldn't be surprising that different viruses infecting the same region cause the same symptoms. Your nose isn't that versatile. It can't turn different colors, or emit different musical notes, for different viruses. All it can do is produce mucus and become inflamed.)

That said, cold viruses are also very well adapted to humans -- they have gone through millions of generations of evolution, adapting to us -- and as a result, they are fairly good at avoiding the immune system, too.