Happy bacteria day to you
You might reconsider your relationship with birthday cake after encountering a study titled: Bacterial Transfer Associated with Blowing Out Candles on a Birthday Cake. On average, blowing out the candles increased the amount of bacteria on the frosting by 14 times.
But in one case, it increased the amount of bacteria by more than 120 times. So the spittle-borne bacteria transfer varies hugely. The study author, Paul Dawson, concludes that it's not really a big deal, claiming our mouths are teeming with bacteria, most of them not harmful.
Dawson has also heard from people with germ-proofing the birthday candle blowing process. A patent, for example, exists for a "Sanitary birthday cake cover and candle system," consisting of a cake holder and cover with holes for candles. (Source: The Atlantic)