Poly folk tend to be very concerned about STDs; common best-practices are to use barriers with new partners (or all partners), get tested for new infections regularly (usually monthly), and to require one's partners to do the same.

OK, but IMHO there is significant risk from infectious agents for which we do not yet have reliable affordable tests or that we do not yet believe to be sexually transmitted or that we do not yet believe to be particularly harmful.

(The spirochete that causes Lyme disease would be an agent of the second category.)