Hi Phillip, 
Good question. There are many factors that may contribute to fungal infection: high humidity, lack of water, too much fertilizer, stressed grass due to being mowed to low, or just changing environmental conditions. Heat and moisture are the most common factors that are favorable to fungi.

What you want to do is to eliminate the factors that allows fungi to thrive. But before that, we need to id the culprit.

Please take a look at this chart which has a great list of diseases, their signs& symptoms, and management strategies!
GOLF - TURFGRASS DISEASES - INFORMATION CHART (osu.edu)

In case you have brown patch, please see this factsheet from OSU's Ohioline.
Brown Patch on Turfgrass | Ohioline (osu.edu

For non-chemical treatment, you can try the following to prevent infection:
1. Water in the morning so the lawn has time to dry.
2. Use a sharp mower blade because a dull blade makes jagged edges thus more opportunity for fungus to enter.
3. Remove thatch so that there is less organic matter for fungi to "eat".
Lawn Fungus: What Causes It And How To Deal With It? - Gardening Tips And Tricks (gardeningelsa.com)
To treat the infection you have, you can use a fungicide that is specifically made for lawn. Test a small patch before applying over your entire lawn; you don't want to kill your grass inadvertently. Whichever product you use, make sure to follow the label for the best results as well as for your own safety. This website has a good list of product recommendations.
5 Best Fungicides For Lawns - Prevent & Control Turf Fungal Diseases (backgarden.org)

I hope the above helps.