Hop Step Jump!.

Tsuruoka Itto, unmarried at 32 years old, just started a job as the janitor at an all-girls' high school, where he's found the girl he's sure he's going to marry, Asahimura Sayaka. When he hears a scream out behind the school one day, he finds Sayaka-chan fretting about a frog inside her clothes, a problem compounded by her frog allergy which soon causes her to faint. Neither the teacher on the scene nor Sayaka's friend will touch the exotic-looking pink frog for fear of poison, which leaves Itto to put his body on the line to save Sayaka-chan. When he finally manages to fish out the frog, it's latched onto a cut on his finger and sucking his blood. Itto in turn soon faints from blood loss, and when he wakes up, he's going to find himself a whole new man...