"He gave me his blood." He hesitates perceptibly, but continues. "Unless a person is on the verge of death, vampire blood -- of our type, anyway -- will heal that person, not turn them. It has side effects, however. We don't usually tell people about it, lest they decide it's some grand scientific experiment."

He trusts that Cordelia, of all people, won't decide it's a scientific experiment, and it's clear he expects her to refrain from going around and telling everyone about it if she doesn't have to.