I don't have any research, but did have a friend whose toddler contracted HMF while nursing. The vast majority of adults are immune (unless they have a immuno-compromise). The lesions don't appear where the contact happened anyway. While her family doctor wasn't completely confident, he did say he thought it was unlikely that she would have lesions on her breasts, even if the skin was broken (but helpfully offered that at 16 months, the feverish baby with the sore mouth could very well be weaned at that point - boo!). 

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