There are a variety of parasites that can infect humans, and some of these parasites can reside in the stomach. The stomach acidity is thought to kill some of these parasites. However, there are many different stomach acidity levels, and some parasites are more resistant to stomach acid than others. In addition, some parasites can attach to the stomach lining, where they are protected from the stomach acid. Therefore, the answer to whether the acid in the stomach kills parasites is not definitive and depends on the type of parasite involved.