A Duck Called Francis

Francis was just a normal everyday duck. He went to school, he played with his friends, he loved his family and he just loved being a duck!
But he got very dirty when he played outside and sometimes, when he came back inside to have lunch or dinner, he forgot to wash himself. Most of the time he was lucky that no dirty germs gave him an infection - but one day, when he came home after playing in the mud, Francis forgot to wash!
The next morning, he woke up with a swollen face and such a sore head. When the doctor came to fix him, he told Francis and his mother something very interesting!
Find out what he told them that would change their way of living.