Weak immune system - 12 weeks pregnant

Anyone else feeling that their immune system has been low lately? Since I got pregnant I've been feeling sick all the time. I don't know if it's because my toddler just started kindergarten or because we're travelling more than usual, but every week there's something new: a cold, a stomach virus, etc etc. I just don't feel fit at all which is so annoying. I want to enjoy this pregnancy. Any tips on how to strengthen the immune system?