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At the University of Chicago Scavenger Hunt each year, there’s a blood drive where teams can earn points for donating. Increasingly, there has been on-campus criticism of how this discriminates against categories of students restricted from blood donation, especially men who’ve had sex with men (MSM) even once since 1977. Give me the straight dope: is the FDA still justified in preventing MSM from donating blood? —Patrick Augustine

Excuse me, Patrick. Since when does anybody at the University of Chicago have sex?

But you want a serious answer. Allow me to provide a two-parter:

Q: Is the prohibition against blood donation if you’ve had gay sex even once in the past 37 years justified?

A: No. If a man had sex with another man one time (hey, maybe even twice<>) in 1977 and hasn’t yet come down with HIV—which of course is the issue behind the ban—he’s not going to come down with it now. Your columnist appreciates the value of bright-line rules as much as the next epidemiologist, but no sense being ridiculous.

Q: Is a prohibition against blood donation by male students who’ve ever had sex with another man justified?

A: You won’t get any argument from me.

Let’s review a few facts, starting with the odds of getting HIV from various behaviors:

• From penetrative anal intercourse—1 in 20,000.

• From receptive anal intercourse—about 1 in 200.

• From receiving tainted blood—9 in 10.

You get the picture. In the old days, getting HIV-infectious blood was tantamount to a death sentence. At the beginning of the epidemic, there wasn’t a way to test for HIV in donated blood, and many infections were transmitted by transfusions—1,220 cases in the U.S. by the end of 1987, plus 1,100 in Canada and 1,700 in the UK. As a result, indefinite bans were placed on blood donations from MSMs in much of the world.

Drastic? Maybe, but together with crucial improvements in screening, it had the desired result: the blood supply was secured. The amount of donated blood that turns out to be HIV-positive is minute—only one unit in 1.5 million flunks screening. From 1999 through 2012, just six cases of HIV from blood transfusion were reported. Given an estimated 13 to 17 million units of whole blood donated per year, that suggests you have something like a 1 in 35 million chance of becoming infected with HIV from a blood transfusion.

Rescinding a lifetime MSM ban would have consequences. One study estimated that if France’s lifetime ban were changed to exclude only MSMs reporting more than one partner in the past year, transfusion-related HIV cases would increase 370 percent. The French idea, you’ll notice, is that monogamous MSMs (i.e., those with one partner in the past year) would be allowed to donate.

In contrast, in most U.S. research I’ve seen, the point of comparison to present practice is a hypothetical donation ban for MSMs with any same-sex partners in the past year. In other words, assuming you’re not also into women, you’d have to be celibate. Possibly due to this stricter standard, two U.S. studies have found a one-year ban would result in a lesser increase in transfusion-caused HIV cases: 8 percent in one study, 66 percent in the other.

What does that mean in practical terms? Yet another study estimates a one-year ban would result in roughly one additional case of HIV per year in the U.S. On the face of it, that would seem to make the present lifetime ban difficult to justify.

But the change doesn’t amount to much. Insisting on what is, for all intents and purposes, a year of celibacy for MSMs drastically reduces the donor pool. Studies in the U.S. and Canada have generally found that moving to a 12-month ban would only allow about 1 or 2 percent of MSMs to donate—139,000 potential donors, according to one estimate.

Would it make more sense to adopt the French notion of allowing monogamous MSMs to donate with no special restrictions? For the general population, arguably yes. In the U.S., the prevalence of HIV among gays is substantially higher than among straights; nonetheless, healthy monogamous gay couples are at no greater risk of contracting sexually transmitted diseases than healthy monogamous straight ones.

But such a rule wouldn’t make much sense for college students, or for people under 30 generally (I pick this age arbitrarily), for whom monogamy isn’t necessarily the default state. Most would thus still be excluded from donating; of those who aren’t, the one partner they’ve had in the past year might be that wild man they met last week.

In short, whatever restrictions short of a lifetime ban one might reasonably adopt for the more settled (read: older) portion of the population, a one-year celibacy requirement (admittedly, when you’re in your early 20s, this might as well be a lifetime) would still be the best policy for young gays.—Cecil Adams