In 2016, a group of Clemson University researchers decided to see just how gross your friend's double dipping habit really is. First, they dipped a bitten chip into a glass of water. According to Science Alert, the water that had been contaminated by the bitten chip "contained 1,000 more bacteria per mL of water than the one exposed to the whole chip."
"That's fine," you say. "I wouldn't dip a chip in water." Hold up, friend. Take a closer look at your salsa. It's got plenty of water. The bacteria nightmare is upon you.
The Clemson team tested some of our favorite sauces to see how they fared post-half-eaten-chip dunks. Salsa, cheese dip, and chocolate syrup all contained more bacteria. But the salsa got hit the worst with "more than five times more bacteria compared to the other two sauces." Why? Without viscosity, or as we like to say, gloopiness, more bits of dip are likely to fall back into the bowl. And as Paul Dawson, leader of the study, explains, the dip bits bring "bacteria from the mouth of the double-dipper" along with them. Ew.