3 weeks ago stomach virus lasted 7 days, lost 14 pounds. 1 week ago a head cold, still hanging around. doctor drew blood, what all will they check for?

Not sure: Neither stomach virus, nor head cold usually require testing, but i suspect there may be more to your story that was not relayed. Your doctor may be concerned of possible other causes for your troubles. You can also ask him what is being checked by the blood tests ordered.

Could be # of things: If you have additional symptoms and findings that would warrant checking out your immune system further, like checking for secondary bacterial upper respiratory infection, and what's accounting for the 14 pound weight loss (diarrhea, constipation, not eating, etc.) would lead your doctor to look at other causes that may significantly impact your getting better like strep, mono, ck cbc, chest xray.