Phase Variation
Phase variation is how some germs quickly change their outer surface so the body's defenses can no longer find them.
Think of it this way
A jar holds small colored balls. Some are red, some blue, some yellow. Your immune system is trained to grab only the red ones. Some balls swap their outer coat from red to blue, so the immune system reaches in and finds nothing it can catch.
Explaining phase variation by grade level
Some germs that cause ear infections can change how they look on the outside. Think of it like a germ putting on a new disguise. Your body learns to spot one disguise, but the germ switches to a new one. That makes it harder for your body to fight the germ off.