Simplify the following text for a Grade 9 native English-speaking person.
Original: The disease can last initially from 10-20 days and can rebreak when the dog is put into a stressful situation which puts stress on the dog 's immune system .
Simplified: The disease can last 10-20 days and can come back when the dog is put into a stressful situation which puts stress on the dog 's immune system .