 Your people's farts are trying to un-alive you, or so your brain thinks. When you smell another person's fart, your brain recognizes it as a foreign object that's trying to harm you. And your brain's not entirely wrong. A fart can carry harmful bacteria and poop particles that can lead to tonsillitis and flesh-eating disease. Luckily, underwear and other clothes will typically protect you from being affected by someone's flatulence. However, when you smell your own tooth, the bacteria you smell is familiar to you, which may lead you to somewhat enjoying it.