How many strains of common cold are usually active at once?

To couch it in my reasons for asking, if on the last night of a ten day vacation far from home, I start to feel cold symptoms, come home and battle a full fledged cold, and four days later interact with my friend and my cousin’s husband, both of whom are suffering symptoms of their own, how likely is it that I’ll have to go through all the bullshit of the past week all over again?

IIRC cold viruses mutate easily and so, when you get a cold you are indeed immune to that particular variety but there are plenty of more around that you haven’t developed antibodies for yet. That’s why there are no cold vaccines – they would work even worse than flu vaccines with their annual guess about which strain will be most prevalent this year.

So, yeah, you will likely fall prey to the new viruses you were exposed to.

Why do could viruses mutate so easily?

Further questions, hopefully not hijacking spirit of OP:

Would the immune system response (or the “severity” of the cold) vary if OP was simultaneously exposed to the original cold strain and friend’s strain (Assuming that they are separate strains)?

Would the immune system response change if OP was continuously exposed to new strains every day for a few weeks? (would the effect feel like a never-ending cold?)

Assuming they both live in the same town and have both caught colds within a 4-day window, what are the chances that they are, in fact, separate strains?

If OP infected friend, and the cold strain mutated, can friend re-infect OP with the mutated strain, or is it likely that it is too similar to original strain?

If most colds are separate strains, why are the symptoms and duration virtually identical when I catch a cold.

Because they belong to the same family of viruses.

The symptoms are mostly due to the body’s immune response, so the answer is more likely because the body is fighting them the same way.

I would have no idea why they would be easy to mutate. Some genotypes are more changeable than others. Wiki says there are over 99 serotypes with little to no cross protection among them.

Are they all that easy to mutate? Or do viruses just evolve quickly because of how quickly their generations occur?

Ie each generation has similar levels of mutation in viruses as in other forms of life (well, though viruses are pretty borderline when it comes to life) but during the course of your individual cold so many generations occur that there’s plenty of time for a mutation to occur.