In my recent book God Is, in the course of responding to a charge that non-acceptance of the HIV causation of AIDS amounted to malevolence or stupidity or both, I used the illustration of a latex condom stopping an HIV virus being like trying to stop a mosquito with a chain link fence (given the size of the virus and the size of the condom’s “mesh”). I also recall using that illustration many years ago somewhere in Credenda also.
In a recent correspondence with a UK med student, he used an illustration that was more to the point. He said it was more like trying to stop malaria with mosquito netting. The mosquitos aren’t malaria, but they are the carriers, and stopping the carrier is all that is necessary. The mosquito netting doesn’t have to be fine enough to catch the malarial infecting agent directly, which is not how the infection comes anyway. In a comparable way, the latex condom doesn’t have to stop the HIV virus directly; it is sufficient to stop the carrier. I will adjust any future use of this illustration accordingly, and I stand corrected.