ACHOO Syndrome is the somewhat strained acronym for "Autosomal Dominant Compelling Helio-Opthalmic Outburst" Syndrome. Doctors who don't care for cute acronyms simply call it Photic Sneeze Reflex, or PSR.
Photic Sneeze Reflex causes people to sneeze one or more times when suddenly exposed to bright light, for example, when walking from a dark room out into bright sunlight. It is an inherited trait, thus the "Autosomal Dominant" part of the acronym, affecting about 25% of the population. The number of sneezes is also genetically determined, and is usually two or three, but can be as many as 43. It is not particularly dangerous, unless perhaps you are a fighter pilot. (If you find yourself suddenly flying into the sun in a hostile situation, the last thing you need is a bout of uncontrolled sneezing.)
WHO GOT IT RIGHT: JP Weigt, Kristy Morgan, Robert Walker, Bob Milligan, Marika Thiessen, Robin Campbell, Andie J, Yael Reuben, Ann Lohry, Suellen Toudt, Scott Flaxman, Marc Quinlivan, David Evans, and Ilana G.