I know it's kind of beside the point, as I know this is thought experiment primarily, but regarding blood donation: one, you may be able to specify a recipient for your blood, but unless it's a match (and blood type matching is only the beginning of the matching process), it wouldn't be used, and two, donated blood lasts a maximum of 6 weeks, so "some point in his life," isn't quite right. You'd have to arrange your threatened infectious donation just before your friend or family member has a high risk surgery or is hospitalized with major trauma.