According to recent research, blowing all over a birthday cake puts germs on that birthday cake. Pretty obvious when you think about it. Would you ever say, "Friend, please blow on my sandwich?" That would be gross, wouldn't it?
Covered in germs in 3...2...
But do people really get sick this way? I searched PubMed for the words "birthday cake disease transmission" and actually found one relevant publication, a letter to the editor of the Canadian Medical Association Journal written at the height of the H1N1 influenza epidemic.
It described "a colleague" extinguishing his 50 birthday candles one by one in a cup of water. "In the age of pandemics, do you really want someone to forcibly exhale over an entire birthday cake that you are about to eat?" the colleague asked.
The scientist behind the recent study doesn't seem too concerned, though. "It's not a big health concern in my perspective," he told the Atlantic. "In reality if you did this 100,000 times, then the chance of getting sick would probably be very minimal."