The Solar Queen Series

The Solar Queen is a series that was written over several decades by several authors. The series follows space age Free Traders that explore alien planets in order to find new trading partners and exotic goods. I have read a couple volumes in this series. I found this list on Wikipedia of the others books in this series in hopes that I might read the others in this series.

Sargasso of Space (1955, as by Andrew North; reissued 1957 as an Ace Double with The Cosmic Puppets by Philip K. Dick)
Plague Ship (1956, as by Andrew North; reissued 1959 as an Ace Double with Voodoo Planet) – complete text via Project Gutenberg
Voodoo Planet (1959, as by Andrew North; issued only as an Ace Double, first with Plague Ship and in 1968 with Star Hunter) – complete text via Project Gutenberg
Postmarked the Stars (1969)
Redline the Stars (1993), with P. M. Griffin
Derelict for Trade (1997), with Sherwood Smith
A Mind for Trade (1997), with Sherwood Smith

Sargolian Creatures

Sickness Hits the Crew of Solar Queen

"It walked erect on two threads of legs, with four knobby joints easily detected. A bulging abdomen sheathed in the horny substance of a beetle's shell ended in a sharp point. Two pairs of small legs, folded close to the much smaller upper portion of its body, were equipped with thorn shack terminations. The head, which constantly turned back and forth on the armor plated shoulders, was long and narrow and split for half its length by a mouth above which were deep pits which must harbor eyes, though actual organs were not visible to the watching men. It was a palish gray in color—which surprised Dane a little. His memory of the few seconds he had seen it on the Captain's desk had suggested that it was much darker. And erect as it was, it stood about eighteen inches high. The creature was able to turn color that matched anything it happened to be sitting on, thus making itself practically invisible."

It was because of this creature that most of the crew became sick. The sickness would start with a headache and then spasms with acute pain. The suffers would then go into a coma like state.

Plague Ship, Andre Norton