Hi Gay
Thank you for your gardening question. Thank you for sending a picture that is always helpful. Unfortunately I can't give you a definitive answer, but I want you to do some more investigating around your plants. Garden Phlox can live in full to part sun environment. It prefers moist well drained soil. 
How is your soil draining? Are the plants getting adequate water? 
It does not look like your plants have powdery mildew, though while working in them you may want to make sure that they have adequate air circulation. 
You stated you were concerned of your other plants, I don't think this is an issue with the picture you sent, but I would pull away some of the plants near the hosta so it has adequate space to grow and any others that the phlox may be impeding on. 
Next question is has anyone sprayed any herbicide in the area that could have drifted onto plants?
Phlox is susceptible to some other insects that could cause curling of leaves. 
Look for spider mites, aphids or phlox plant bugs. Do your leaves look like the coloring is stippled?
Look at the underside of the leaves, and on the stems. If you find one of these, make sure to accurately identify. You can send back a picture if needed.  I have attached several articles to help you. 

https://ask2.extension.org/kb/faq.php?id=796894

https://apps.extension.umn.edu/garden/diagnose/plant/annualperennial/phlox/leavesdistorted.html
https://extension.umn.edu/flowers/tall-garden-phlox#spider-mites-818611 https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/insects/plant-bugs/phlox-bugs

https://mortonarb.org/plant-and-protect/trees-and-plants/garden-phlox/#care-knowledge