You're kidding right? yes they are expensive and take up tremendous amounts of time and resources, but haven't you heard of all the 'tainted blood scandals' that people have died from? In Canada anyway a person goes through an extensive
Interview and screening process (I think it is 8 steps) each and every time they go. Each and every donation is tested for infectious diseases including sexually transmitted diseases, hepatitis, hiv 1 and 2, and west nile virus. A person may be infected by these and not know it, hence they might still be honestly thinking they don't have anything, but the tests show they do, and their doctor is informed.