A lawn disease, or otherwise known as fungus, is caused by excessive moisture. Thatch is the main culprit for this problem. Thatch is long dead grass that resembles a bad "comb-over". It often happens when you don't cut your lawn short at the end of fall and during the course of winter that grass folds over and creates a pocket where oxygen doesn't enter and moisture is trapped inside. Over the course of these moist months, the trapped moisture causes the soil to rot and the roots develop spores or mold that grows deep into the soil.
Prevention: Cut your lawn short at the absolute end of fall, right before we get the first snowfall. If you missed the opportunity and notice that your lawn looks matted over in the late winter/early spring, Power Raking is very effective for removing excess thatch, dead grass, and debris left over from the fall. Another tip: make sure your irrigation program doesn't have major leaks in the sprinkler heads or the main lines.
Treatment: Lawn fungus is not an easy fix, prevention is key! With that being said, if you have it, save your energy trying to cut it out these patches because the real problem is in the roots. There are products that can actually cure the disease. It takes a long time, usually 1-2 years to effectively eliminate. Soil conditioner or Humates can help break down the clay and allow some relief, but the real source is through commercial grade fungicides.
Here are some common images of lawn fungus...