I was the first one in my family to get the flu. It was disgusting, but it was (mercifully) over in 24 hours. We disinfected the bathroom, and everything else I had touched. Everybody washed their hands again and again. But to no avail.
Elizabeth (youngest daughter) was the next victim, then John. John's symptoms weren't as severe, and he recovered faster than the rest of us. He continues to have a strong immune system. However, in some ways, he didn't recover for two weeks, and that's why I'm writing this section.
Within a day John was eating the same foods he ate before, and his stools were normal, but his behavior was off the scale! The simplest request would elicit an explosive tantrum. This continued for a week - maybe two. I don't believe his diet changed in any significant way after the flu, but something was amiss. He was reacting to everything. Dairy seemed to evoke nasal allergies, and we hadn't seen those symptoms in months.
I did some research and discovered that the flu, or any other form of intestinal distress, can strip away the enzymes that are needed for digestion. This is especially true of lactase, but it holds for all enzymes, to varying degrees. It takes days, maybe weeks, to rebuild those enzymes. After the flu, John was unable to digest his food completely, and the (still unidentified) pathogens thrived on the excess carbohydrates. In retrospect, we should have cut back on carbs and avoided dairy for at least a month, giving him time to rebuild his store of enzymes. I didn't know any of this at the time. Of course he was eating cereal and milk almost every morning. :(
If nothing else, this episode demonstrates the undeniable colon-brain connection. If he isn't digesting his food properly, he's not the same boy.