Mcgarrett is a liver transplant recipient and he would be on any rejection drugs for that.
His immune system would have been compromised and he should have been the sickest one from the virus.
McGarrett makes a mistake that no military person would make. When the team sends the tissue samples up to Noelani on the helicopter, Steve ends the radio call with "Over and Out". The correct way to end the conversation is to just say "Out". By saying,"over and out" basically you are telling the other person that you are waiting for their reply (over) but have just turned off your radio (out).