Search - Charon: A Dragon at the Gate (Four Lords of the Diamond, Bk 3)

Somewhere from among the four human-settled worlds of the Warden Diamont, hostile aliens were spying on Earth. But no agent could be sent to investigate and report back. All trips to the Warden worlds were one-way. There, a microscopic symbiont invaded all life forms, after which life became impossible outside the Warden system. THe same organism destroyed any for of machinery, so no message could be beamed back by normal means.
Book three of four