The “infected gammarus” (scuds with an orange spot on the back or belly) are actually parasitized by a microsporidian parasite. The infection causes the gammarus to develop a bright orange patch, making them more visible and easier prey for trout and grayling.
For fish, this is like a “target signal” — they key in quickly on these vulnerable scuds. That’s why orange-spotted gammarus imitations are so effective in fly fishing.