What are some of the most commonly undiagnosed illnesses?

Dr. Neel explained that more than half of people get mono by the time they graduate high school, but most of them will never know it, because they have only mild symptoms.

“But in some cases, mono can wipe you out for months. Sometimes it can look like a really bad case of strep,” he says, and brings us to another common illness that’s often missed—Strep.
Due to so many false negatives in the rapid strep tests, it also gets missed often.
In many cases, especially with children, Dr. Neel says the strep patient will report having a headache a day or two before coming out with strep.
“I don’t know why,” Dr. Neel said, “but I’ve seen that over and over again. Common illnesses don’t always present themselves the same way.”
On this note, Dr. Neel adds a medical saying that it sometimes seems “there are more uncommon presentations of common diseases,” than the other way around.
“I get some strep patients that don’t even have a sore throat,” Dr. Neel said, “but then you look in their mouth and you see all these white spots, and their throat looks like it’s on fire.”