Blowing candles on the birthday cake is one of the traditions which remained constant over the years.
Aside from the threat of getting spit on the icing (ewwww), according to a recent study, blowing out candles on a cake can increase the number of bacteria on said cake by 1,400 per cent.
They were then asked to blow them out, before scraping the frosting off the fake cake and spreading it on agar plates for the bacteria to grow.
Scientists from Clemson Universiy, South Carolina, got a group of students to eat a piece of pizza "to help the salivary glands get going" before lighting the candles on a cake-shaped piece of Styrofoam.
On average, blowing out candles resulted in 15 times more bacteria on the icing than when candles were not blown out. The cakes with the blown out candles also had more types of bacteria than the cakes without candles. "Some people blow on the cake and they don't transfer any bacteria".
And in one disgusting case, the number of bacteria increased more than 120 times - suggesting that some of us transfer more germs than others.
Dawson added that while some people transfer a ton of bacteria, others don't transfer any.
But a new study might make you think twice about the decoration, or eating a slice after they've been blown out.
"It's not a big health concern in my perspective", study co-author Paul Dawson told the Atlantic. However, it seems this habit is not so hygienic, as blowing air above the candles and the cake is not among the cleanest things in the world.
In theory you could be spreading airborne diseases such as the flu by blowing out candles on a birthday cake. Some people might transfer a lot of bacteria while blowing, and others might not transfer any.