Because it was an allergic reaction by Jim's body. Not everyone is allergic to the same things. Where they blundered is with showing whatever that chemical compound was, directly affecting the plant that they suspect gave them their powers. It contravents the allergy angle. Though it's possible that Jim is naturally allergic to the chemical anyhow, allowing the chemical to directly affect the altered genes in his body. Not to mention Stephanie's accelerated healing.
What I want to know is, why the watcher needs constant "treatment," while the Powells seem to be doing relatively okay. Also, does Jim also have some kind of healing factor as well? The bullet wound on his neck is gone, and the cut on his hand went away, once his powers came back.