His center also offers shots against what it can fight: flu and meningococcal meningitis, a rare but sometimes dangerous infection known as “freshman meningitis.”

To make it safer, the evening is overseen by student sobriety monitors and decorated with hand-drawn signs — of the ilk that usually say “Beat Cal” — but bearing slogans like “Consent is Sexy.”

But the most crucial role is played by the “peer health educators” who live in each dorm.

They meet with freshmen before, and ask any with cold symptoms to feel free to watch, but not to kiss anyone.

And they teach safe kissing.

“We tell them, ‘Don’t floss beforehand, don’t brush, don’t do anything that could create microabrasions in your gums for germs to get in,” said Michelle Lee Mederos, a former educator who graduated in 2011. “And we have tables where we offer mints and little Dixie cups of mouthwash.”

Mouthwash is the main line of defense. This year there were two tables dotted with paper cups, along with condoms and other sexual health supplies.

The educators also have square latex dental dams to offer, “but they’re generally made fun of,” said Ms. Mederos, who used them in dorm demonstrations of how to safely perform oral sex on a woman. “In two years of ordering supplies, I had to order them only once — for those demonstrations.”