If you're not familiar with Japanese culture, certain things about anime may come across as confusing. One of those things is sneezing. You know how anime occasionally interrupts a scene to depict an unrelated character sneezing? Yeah, there's a reason for that.
In anime, people sneeze for the same reasons that people do in real life - illness, allergies, random irritation, and so forth. While this can add a touch of realism, these explosive animations rarely contribute to the over all plot. However, sometimes these instances serve a purpose that has little to do with why people actually sneeze, and everything to do with superstition.
In parts of Eastern Asia which include Japan, Korea, China, and Vietnam, a sneeze means that someone is talking about you behind your back. Sneezing once implies a compliment, while sneezing twice means that what they're saying isn't so nice. Sneezing three times means that someone is secretly in love with them, and any more than that means they're coming down with a cold. In anime, the full superstition usually doesn't make an appearance, however - most of the time, the talked-about character sneezes once regardless of what's being said.
Sometimes, the trope is flipped on its head - characters will wonder aloud if someone is talking about them when they're actually getting sick, or they'll insist they're catching a cold when the real issue is their best friend talking smack about them in the next room.
So, the next time your allergies are acting up or you're feeling under the weather, just pretend you're an anime character who's important enough to have people talk about you when you're not around. It won't exactly solve the problem, but at least it's fun.