What pecentage of the population are infected with the common cold?

Depends: During cold seasons (typically summer and winter), infection is very common. But in between cold outbreaks, infection is less prevalent. Cold viruses don't cause chronic infection, so it's not possible to give you a number.

Rhinoviri Strains: There are hundreds of strains of the rhinovirus that causes the common cold. The good news is that you will develop immunity to the strain you have that gives you cold symptoms. The bad news is that there are hundreds of strains so most people are susceptible to many strains they have not been exposed to.