I would suggest changing the soil ASAP.. to do this, you will have to make sure the soil is very dry, and tap the pot a few times to loosen the soil, sow gently pull the plant out and the soil should come with it. Tap off the most soil you possibly can and discard the soil. Repot the plant. If you are using store bought potting soil it should be free of contaminants. Yet, if you are using soil from outside or some sort like that, you will need to bake the soil in the oven at low temps to remove any bacteria/parasites and the like. Baking it at 100 farenheight for 30 mins should suffise. Also make sure you wash out the pot you are growing in with a mild bleach solution to kill any leftover parasites. After using this technique, this problem should not reoccur. Good luck, and happy growing!