Subtitles section Play video Print subtitles Mmmmhhhh, puppies! Most dog-owners say companionship is the number one reason to have a cuddly, devoted tail-wagger, and some people also have dogs for assistance or protection. And just look at them! They're so cute! So it's not surprising that we're willing to put up with less-savory features of dogs, like muddy paw-prints and slobber all over everything. Yet slobber and other pet dirt may actually be a pet benefit too, especially if you happen to be an unborn baby. Dogs - and also cats - influence the microbial/bacterial communities in our homes so much that if your mother lives with a dog or cat while she's pregnant with you, you're about 30% less likely to suffer from allergies as a child. This sounds kind of crazy, and we don't know exactly why it happens - but the most likely explanation is called the "hygiene hypothesis." You know how children from Amish farm families have been found to suffer less from allergies and asthma than is typical in the modern westernized world? Well, scientists think it's because their immune systems develop more fully thanks to exposure to a wide variety of dirt, bacteria, and germs in fermenting feed, cow manure, and other barnyard delights. A key part of your immune system is the cells that recognize and neutralize foreign bacteria, viruses, transplanted body parts or even your own damaged cells. Healthy cells in your body have distinctive proteins on them that immune cells recognize as part of 'you', while intruders and unhealthy cells without 'you' proteins are flagged for careful monitoring. If any 'not-you' stuff starts causing too much harm, your immune cells will attack it and take note to act quickly and vigorously against it in the future - basically, learning who's harmlessly passing through, and who's a dangerous intruder. But if the immune system incorrectly identifies an intruder or doesn't properly learn who's who in the first place, our bodies can overreact to harmless substances - like a life-threatening allergic reaction to a minor bee sting. In the Western world, the percentage of children who suffer from immune-system overreactions like allergies and asthma has roughly doubled in the past 40 years or so, even as infectious diseases have become much less common thanks to improved hygiene, water & sewage treatment, and so on. It's highly likely that the increased prevalence of allergies and asthma is due in part to the fact that the environments we live in are too clean and don't give our immune system proper opportunity to learn who's who at a young age. Kind of like how we're better at learning foreign languages when we're younger, our immune systems are best at learning to distinguish harmless foreign substances from harmful ones when exposed to them very early in life. That's why having dog-slobber, kitty hairballs and muddy pawprints around your mother while you're in utero might get your immune system off to a proper start even before you're born! We still don't know exactly how your mother's exposure to extra bacteria influences you in the womb, but we do know that having a pet around before - and after - your birth may help keep your immune cells from barking up the wrong tree. This video was brought to you by... you! Those of you who've made a recurring commitment to MinuteEarth on Subbable are now paying for the illustration & animation of all our new videos, and for that, we'd like to say Thank You. Imagine if every MinuteEarth video since August had no... video! Our next goal is for monthly subscriptions on Subbable to also cover the costs of research and script writing. Because without content, MinuteEarth might just become a slideshow of cute puppies... which wouldn't be so bad, but then again, it wouldn't be MinuteEarth. So please, help us make more videos by becoming a recurring subscriber on Subbable. You can click the link here or in the video description, or go to subbable.com/minuteearth.