I think "this" refers to having toxoplasmosis and developing antibodies. But it's hard to say for definite; I think the paragraph the sentence is in may be poorly organised. It's especially confusing because of what we know about the real world: we know that the population of a country like the UK is likely to be roughly 50% male and 50% female, but the first clause mentions 80% of the whole population, and the second clause mentions only women.
Do you have the rest of the paragraph for us to see?
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