Do you remember the leafy greens ban back in January, or the romaine ban that happened back in April? No? What's different?
From my research, detailed in the short book I just published, it is clear that microcephaly is a cross-reaction between two viruses.
Short answer: very, very rare. Longer answer: not as rare as we'd like.
In full disclosure, I do drink a cup of Yerba Mate tea most mornings.