Well, the endemic malaria travel exclusion is based on the life cycle of the malaria pathogen (various species of Plasmodium). Each of the little monsters live in a red cell for a period, using the hemoglobin therein as a food source (red cells are a Ziploc baggie of hemoglobin, that being the complex red-colored molecule responsible for shuttling oxygen from the lungs to everywhere else).
So receiving a transfusion from a not-yet symptomatic (or at least undiagnosed) donor infected with malaria is a really good way to instantly contract the disease. And post-donation serologic tests are far from perfect for malaria.
Other exclusions, since you ask: personal history of most cancers (risk of transfusing CTCs, circulating tumor cells); exposure to other infectious diseases (same rationale as malaria); transplants of dura and the like (prions, iirc); use of any of a long list of medications e.g. blood thinners or potential liver toxins. (Bear in mind that recipients are always frail and often profoundly immunosuppressed)
Gay men still have a blanket exclusion, I think, which was appropriate thirty years or more ago while HIV was being sorted out. Now? Uh, no (Did I mention I’m not a huge Red Cross fan?).
Self-reported risky personal behavior (on a confidential questionnaire) will exclude donated blood: being incarcerated for >72 hours in the last ?year?; IVDA; cash transactions of a certain personal nature.
Finally, exclusions can be made wrt donor safety: being too small or light to safely, suddenly part with a pint of blood; having a borderline red cell level (tested on-site immediately prior to donation); significant heart or lung disease; poorly controlled hypertension (also tested on-site)
You did ask… 
-sutton
happy to drone on at length re hemoglobin. Or platelets, for that matter.
Miraculous things…
