Opportunistic infection
Okay, let's say that your body is a big house, and inside the house there are lots of doors that lead to different rooms. These doors have locks that only certain germs can open. Some germs have keys to certain doors, but can't open other doors because they don't have the right key.
Now, sometimes when you're sick or your immune system is very weak, some other germs that don't usually have keys to any doors sneak in and try to open them. These germs are called "opportunistic" because they take advantage of a moment when your body is weaker than normal to try to cause an infection.
When an opportunistic germ manages to open a door and get into a room, it can make you even sicker than you already were. This is why it's important for your body to have a good immune system, so it can fight off all these germs and keep them from opening any doors they shouldn't.
Now, sometimes when you're sick or your immune system is very weak, some other germs that don't usually have keys to any doors sneak in and try to open them. These germs are called "opportunistic" because they take advantage of a moment when your body is weaker than normal to try to cause an infection.
When an opportunistic germ manages to open a door and get into a room, it can make you even sicker than you already were. This is why it's important for your body to have a good immune system, so it can fight off all these germs and keep them from opening any doors they shouldn't.