The causal intervention is really an act of exposure.

If you expose once, or expose until infected, or expose but exclude those not infected, then the difference between infected and not-infected populations is obscured; the reason some individuals were infected (or not) from a single exposure needs to be explained. If it isn't, then I can say that part of any difference between the infected and not-infected populations is due to whatever factor made some of them fall prey to the infection on one exposure.