Humanae Vitae has, in its history, been challenged by events. There have been debates about the use of condoms as prophylactics and the morality of contraception in irregular sexual activity. But the outbreak of Zika in the (southern) Americas presents us with an altogether more direct problem. Zika is an infection caught from the Aedes aegypti mosquito. It is strongly suspected, though not fully proven, that, in some pregnant women, it causes microcephaly in the foetus. (Microcephaly is a severe shrinkage of the brain, which damages brain function.) The World Health Organisation predicts that some four million people in the Americas will be infected with Zika this year. And here lies the problem. The obvious and sensible precaution is to avoid pregnancy until the situation is under control. Unlike other attempts to circumnavigate orthodox doctrine, this precaution is explicitly and intentionally contraceptive. The intention may be benign but the contraceptive action is held to be intrinsically evil. This is emphasised in Humanae Vitae, echoing Casti Connubii, “(It) is absolutely required that any use whatsoever of marriage must retain its natural potential to procreate human life.” Bearing in mind that absolute commands may be hostages to fortune, the Hierarchy face a dilemma. Once it is accepted that artificial contraception is justified by benign intention, we open a gate we cannot close. But the options are not attractive. Can we imagine the likely effects of proclaiming that married couples should refrain from sexual activity for an indefinite period? Might presenting this on the grounds that no contraceptive method is perfect be seen as disingenuous? Should all couples in the 21 countries at risk immediately master and use natural family planning? Or is it enough to keep quiet, and hope that no one asks any questions?