Interestingly, this would make sense in the fringe theory posted by another commenter on this thread. If flu viruses fall to Earth from space somewhere in Earth

Interestingly, this would make sense in the fringe theory posted by another commenter on this thread. If flu viruses fall to Earth from space somewhere in Earth's orbit, then it would simply be at some point in the orbit where it is winter in the northern hemisphere and summer in the southern hemisphere, confirming your observation that the flu seems to be more prevalent in the summer months of Brazil.