HIV is a fragile virus.  According to studies done by Harvard's medical school and the CDC, HIV can only be passed via blood-to-blood contact or contact between blood and a bodily fluid that carries the virus.  This usually happens via injection (penetrating the flesh with a needle that has been used on a contaminated person and has come into contact with the virus), blood transfusion, or sexual contact (when the sexual contact involves transfer of contaminated fluids).  HIV does not care whether you are a drug abuser or a DARE advocate, a tart or a prim-and-proper monogamist, a perve or a missionary-position vanilla; HIV only cares about one thing: staying alive in a host.  Anal sex is risky only because the anus is very thin (it has the consistency of wet paper towels) and easily torn; vaginal sex likewise is risky when the vagina has fissures in it for whatever reason (herpes sores, tearing from too much friction, the penis has a cut or sore on it, etc).  Oral sex is actually pretty safe unless the mouth has sores or cuts.  Saliva and stomach acid are both hostile to HIV and tend to kill the virus.

Blood drinking is no more risky than oral sex.

The problem, of course, is that many people do have cuts or sores in their mouth.  Gingivitis is common, as is herpes (most people get cold sores from time to time).  Gums bleed after flossing or after a vigourous brushing.  As if that isn't enough, we also tend to have problems with sore throats, burns (after eating food that is too hot or spicy), cuts or sores from accidentally biting our tongues while eating...This is why oral sex can be such a risky thing, and why blood drinking is likewise an act of total trust and total surrender.  Quite simply, you are putting your life in your donor's hands.  Donors tend to be lovers because lovers are often more willing to engage in such a bizarre and intimate act as "feeding" blood to their partner, and because most vampires and blood fetishists are not crazy enough to trust just *anybody* with their lives.

There are ways to minimize risk.  Don't brush your teeth or floss before drinking blood; wash your mouth out with an antibacterial mouthwash to kill any germs that might cause an infection on contact with the donor, but don't brush until well after you have fed and no longer have traces of blood in your mouth.  Don't drink from a donor who you either don't know well enough to trust his/her cleanliness, or who hasn't presented you with negative results for various blood tests.  Don't drink when you have cuts or sores on your tongue, mouth area, lips, or throat unless you are with a donor who is known to be clean of disease and who preferably is your exclusive partner and does not partake of the bodily fluids of other people.  (Monogamy is a nice idea, but it isn't really realistic.  "Serial monogamy" happens to be the cultural norm, and it's a contradiction in terms -- if someone has had sex or shared fluids with eight other people before you, then you are effectively drinking from all nine of the people involved, plus the sexual partners of any of your donor's previous lovers...Responsible polyamory is just as safe as serial monogamy, and it is a good idea to use latex condoms and other latex protection with anyone other than your primary partner when dating, and to only share fluids with one person at a time; if that person has not tested clean of HIV, herpes, etc, then do not share fluids at all.  Both partners, or all partners if this is a triad or whatever, should follow this basic etiquette.  Fluids should never be shared outside of the nuclear family unit, even if that unit is not a committed, married one).

It would be wise to remember that HIV does not show up on either an ELISA test or a Western Blot until after the incubation period, when it is most contagious; a clean bill of health is not failsafe.  Furthermore, HIV is not the only dangerous and incurable virus out there.  Herpes cannot be cured, it can only be treated.  Hepatitis can be vaccinated against, but the vaccine for Hepatitis B is only 75% effective, and the vaccine for Hepatitis C takes six months to get, in a series of three shots, which are rather expensive and are not always covered by a medical plan (if you live in the USA).  Some strains of syphilis are resistant to antibiotics.  Believe me, you don't want syphilis!

Ultimately, the choice of who to drink from depends on one's comfort zones.  How much trust do you have for your partner? how reckless do you feel?  Drinking from a strange person is not advisable unless you have no cuts or lesions in your mouth/throat area, at any rate; other than that, it probably shouldn't make anyone panic to have intimate activity of that sort with a stranger who you never see again, it's less risky than unprotected sex, but it's not a habit you want to get into.  Blood carries disease.  We can't get away from that; all we can do is minimize risk.  The ideal way to feed is with a steady partner who only shares fluids with you.  If that isn't possible, then don't feed unless you know you do not have openings in your mouth or throat that the blood can enter.

Drinking animal blood is even riskier, thanks to factory farming.  Most blood from large producers is infected with salmonella, often with e. coli and botulism as well.  The human body can actually tolerate small amounts of these bacteria, and continued exposure will build up an even greater resistence to such toxins, but it's best not to push your luck.  Eating rare steak might be safe if you have a cast-iron stomach.  Drinking the animal blood is not safe unless you get the blood from a butcher who got the meat from a small, free-range supplier, and even then it's a risk.  If you hunt the meat yourself (do you have a permit to hunt deer?) and slaughter it yourself, you at least know where the blood came from -- you know it hasn't been slaughtered in an abattoir and processed in an environment that gets animal shit and so on in places where it isn't supposed to go.  (Things haven't changed that much since Upton Sinclair wrote The Jungle.  Really.)  BSE is more of a risk in America than in the EU, because Americans still feed their ranch animals food that is made from reconstituted animal parts.

Don't forget about antibiotics, either (another good reason to avoid factory-farmed meat and dairy products).  Finally, don't drink raw pig blood -- it's risky.  Trichinosis is probably easier to get from raw flesh than from raw blood, but I wouldn't want to take my chances.  On the other hand, black pudding is very nutritious and filling, it satisfies the blood craving, and the blood is cooked, so it's safe to eat.

Do not drink more than a shot glass or so of raw blood -- it will make you puke your guts out.  Blood is very rich and large quantities curdle in the stomach.  (You shouldn't need that much, anyway; and taking more than a shot glass or so from a willing donor is bad for the donor.)  Eating bread with your blood, or mixing it with milk and warming that mixture up, will help somewhat, but blood is so rich that you really do not need to drink much before you fill up.

Feeding technique:  Biting is bad -- the human mouth is germy, and biting is painful anyway; it also doesn't produce much blood.  It is much better to use a sharp, fresh lancet, scalpel, or razor blade on an area of skin that isn't near any veins or arteries or internal organs.  I cut on the back, the upper thigh, the chest, or (very occaisionally and very carefully!) the hand or certain parts of the neck.  Cutting deeply is unnecessary.  What you are trying to do is cause the equivalent of a shaving nick.  It doesn't hurt to accidentally cut yourself while shaving, does it?  And you can get a lot of blood if you don't immediately stanch the flow.  Making two small, shallow cuts in an X-shape prevents the wound from closing immediately and allows you to get more blood from the wound, but it also increases scarring risk.  Never make a circular cut, the skin in the middle of the circle might die once it has been cut off from normal blood flow, and fall off.  Ick.  (Ouch.)  Most cuts of this nature close up in six minutes or so, although an X-shaped cut sometimes stays open as long as fifteen or twenty minutes.  Remember that you are only trying to get the equivalent of one mouthful of blood, at most.  If you start to feel dizzy or queasy, stop drinking, because you have reached your limit.  If you are a trained phlebotomist, then it is safe for you to use a syringe and needle to remove a small amount of blood from your donor's veins, but I discourage this unless you have actually been trained to do it.  It's a painless technique, but it can backfire, and untrained practitioners can cause serious damage (a nasty hematoma being the least you can expect if something goes wrong!)  Have first aid supplies handy (bandages/plasters, disinfectant).  Use them after feeding.  I don't use disinfectant before feeding because it tastes disgusting, and I am guilty of not always using it afterwards, and I haven't seen a wound get infected yet, but it's better to be safe than sorry.  Be ready to use direct pressure, pressure points, or even a tourniquet if an accident happens (unlikely if you are avoiding veins and arteries).  If you don't know the procedures for stopping a heavy blood flow, take a first-aid course.

Vitamin E oil or lotion helps reduce the risk of scarring.