How will isolation affect long-term immunity? Playing dirty: as humans evolved, our immune systems learned to cope with the microbes around us in the natural world. Illustration: Lehel Kovács/The Observer Playing dirty: as humans evolved, our immune systems learned to cope with the microbes around us in the natural world. Illustration: Lehel Kovács/The Observer Healthy immune systems work best when exposed to microbes. So what will lockdown have done to our resistance to germs? Sun 16 May 2021 05.00 EDT Every time you kiss another human being intimately for 10 seconds, more than 80m bacteria are transferred from mouth to mouth. If you’re at a party and double dip your tortilla chip into the salsa three times, around 10,000 bacteria will be transferred from your lips to the dip. Say “hi” to your co-workers as you sit down at your office desk and you’ll also be greeted by over 10m bacteria on its surface.