Just as you learn to recognize poison ivy in the woods, it is important to recognize blooms that could affect you in the water. If you see a green scum on top of the water that looks like a paint spill, do not touch it and do not let children or pets get it in their mouth.
Blue-green algae (also known as cyanobacteria) are microscopic organisms that naturally occur in Illinois lakes and streams. They can reproduce very quickly in warm water where there is dirt, fertilizer and sunlight. Learn about it at http://epa.illinois.gov/topics/water-quality/monitoring/algal-bloom/index
Last year when the IL EPA found a toxin in the blooms in the shallows at Dunlap Lake, the deeper water tested safe. If you choose to be in the water it is probably best to shower well after swimming and not to eat any fish until the blooms have cleared.