Strep throat, like other throat infections, is a common infection. I would recommend talking with your child's doctor
as it is important to understand the exact details.
In general, the body works in balance with bacteria. We all have bacteria living on our skin, in our mouths and in our colon. The balance between the bacteria simply living there (colonizing) versus infecting us is a complex one--something that infectious disease researchers do not fully understand. Some bacteria are a little more aggressive then others--but even the most aggressive can sometimes just "colonize" and not necessarily infect.
The bacteria that causes strep throat is called Group A beta hemolytic streptoccocus. Depending on where you live 12-50% of people are colonized with this bug--that means they have it living in their mouth or throat and not causing infections. So many people have it because it is easily transferred from person to person by air or cough droplets (simply breathing on a neighbor could do it). Hand hygiene is important for other bugs, but probably did not cause this infection. At any given time, these colonizing bacteria can somehow cause infection. It sounds like your child unfortunately had the bacteria infect him as opposed to just being there.
I hope your son is better soon. I would recommend discussing this further with your doctor as strep throat is an important disease to understand--it can have long term effects that one should be aware of. Good luck!