Asked by: Jenny Price, Weston-super-Mare
With each new breath, you take in about 50 potentially harmful bacteria. Virtually all of these are promptly destroyed by your immune system without you feeling a thing. But some bacteria and viruses can successfully infiltrate the body and lie dormant without causing any symptoms for many years until they suddenly flare up.
Chlamydia infections, for example, show no symptoms for 50-70 per cent of women, because they lurk beneath the radar of the immune system. Any disease that your immune system does pick up, however, triggers swelling, inflammation and fever as the disease is fought, so that’s something you will definitely notice.
Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.