There is a fungus going around affecting Impatiens called Downy Mildew. It lives in the soil and is spread from plant to plant through the air. Once a plant is infected there is nothing that can be done to fix it and the plant will die quickly.
If you notice that you have Impatiens (or any plants) infected by Downy Mildew you should rip them out of your garden or pots as soon as possible. DO NOT COMPOST THEM. Tie them in a plastic bag and throw them in the trash. This will help prevent the mildew from spreading.
Plant options similar to Impatiens:
Begonias
Torenia
Coleus
Flowering Vinca
Petunias
Lobelia
Have you noticed anything wrong with your Impatiens?
kjprommel
krisazaveri
tashaiwanski
Morris County Farms, Inc. 
This must be what happened to my impatients several weeks ago. It started in one pot and then spread to another nearby — all died very quickly! My question to prevent the same thing next summer is: should I discard the soil in those pots, or will cold winter temperatures kill the downy mildew spores in the soil? Will cleaning the pots and treating them with bleach help prevent downy mildew next season? Or would I be better off just planting one of the alternative plants next year?
You should definitely discard the soil in the pots, and I would also recommend sanitizing them with a bleach solution. If you want to make extra certain you don’t have downy mildew next year, I would recommend planting one of the alternative plants to be safe.