I don't think it's contagious. It appears you may be in soil that would be considered "hot". From looking up your previous post, you mentioned using premium potting soil. It may have nutrients in it for the plants to have access to, as well as perhaps not as much perlite that a seedling would have wanted. Also mentioned using tap water, I do too! Although, I let it sit out in watering cans for a day before I use it. Seedlings are sensitive, doesn't mean they can't bounce back.