When my nephew was 3 years old he contracted Swine Flu and Bronchial Pneumonia at the same time. They had to put him in a drug induced coma for six weeks to cure it. He lost over half his body weight and before he came home had to have a feeding tube installed so my brother and his wife could feed him while he slept because he could not eat enough when he was awake to gain back the proper body weight fast enough.
It took him a year to after he got home for his immune system to recover and for him to regain his weight and health. Now he is a typical six year old.