Here's another vaguely remembered story from some anthology in the 40s to 70s. This one is about an epidemic of some new disease that hits a military base (off-world, I think) and perplexes the doctors until they do get it under control. A lot of men are incapacitated but I don't think the disease is very deadly. At the end, the doctor who figures it out speaks didactically about how encountering new diseases is inevitable during exploration, and though they were lucky this time, sooner or later they might encounter something they can't handle. In walks a marine(?) with a "Hey, Doc" attitude and a rash of green triangles.
This is a bit of a long shot, as it does not match any detail of your description: "Expedition Mercy", a novelette by J. A. Winter, M. D., originally published in Astounding Science Fiction, November 1948, available at the Internet Archive; reprinted in the paperback anthology Great Science Fiction About Doctors edited by Groff Conklin and Noah D. Fabricant, M. D. A sequel, "Expedition Polychrome", appeared in Astounding Science Fiction, January 1949, also available at the Internet Archive.
The disease in this story is quite deadly, as it killed all twelve members of the first expedition to the planet Minotaur. The story is about the second expedition. What sort of matches your description is that in the end, after the medical mystery is solved, one of the doctors is delivering a didactic little lecture when a crewman walks in with an impossible symptom. Unfortunately, it's not "green triangles":
"Tommy, you will now receive Dr. Edwards' famous indoctrination course on how not to be surprised at exotic diseases. There can't be any new diseases. You see, the human organism is capable of acting in only certain ways. For example, the blood pressure can go up, it can go down or it can remain the same. The temperature can be elevated, it can be subnormal, or it can be normal. And so it goes for every function of the body—it can change only within the limits of its own capacity to function.
"When we study exotic diseases the difficulty, therefore, is to find the causative agent. The disease itself is probably greatly similar to one with which we have been familiar on Earth for hundreds of years."
"Oh, I see," said Tom. "The roads it may travel on might be new, but it's still the same old model that's doing the traveling."
"Exactly," replied Bob. "To give you another example: the body is capable of only certain color changes. The skin might turn brown, due to the presence of melanin, one of the normally found pigments. Or it might turn any one of the colors seen in the degradation of hemoglobin. You know, those fascinating color changes of dark blue to green to yellow and all shades in between that you see in a bruise—or that shiner I saw you wearing last year.
"No," he continued, without giving Tom a chance to explain that he got the black eye from bumping into a door in the dark, "we could never expect to see a man turn an aquamarine blue. There just isn't a precursor for that color in the body. So we'll never see an exotic disease where the skin is aquamarine or we'll never see a disease where a man reacts outside of the normal limitations of response."
"So that's it," mused Tom. "Yes, what is it?" He turned around as a knock came at the door.
It was one of the crew members. "Sorry to interrupt, sir, but I'd like to have Dr. Edwards take a look at me. My skin is kind of a funny color."
Edwards turned around. Like the Bay of Naples on a sunny day, or Lake Superior in July, the man's skin was a beautiful, vivid aquamarine blue.
The sequel "Expedition Polychrome" is about the man with blue skin.