Okay kiddo, so every person has something called a 'compatibility gene'. This gene helps our body figure out which germs or bacteria are good for us and which ones are bad.
Think of it like a secret code that only our body knows. The code tells our body what germs are like friends and what ones are like bad guys trying to make us sick.
This gene can be different from person to person. That's why some people may get sick easier than others, because their secret code doesn't work as well. Some people might not get sick as often because their secret code is really good at keeping them healthy.
Doctors and scientists study this gene so they can figure out how it works and how to help people who may not have a very good secret code. They even use it to help match organ donors to people who need organs so that their bodies don't reject them. It's a pretty amazing gene!