First, Wicca is not about spells. Magic doesn't care what your religion is. Wicca is a religion, and plenty of Wiccans do not work spells, and even more spellworkers are not Wiccans.
Second, depends who you ask. Theoretically, I don't see why not. The fact that medicine can't cure something shouldn't affect whether magic can cure something. *However,* the bigger the desired change, the more energy needed. Helping your immune system fight off the flu a little faster should require far less than stopping a disease that normally doesn't stop. (By that same logic, we're never going to fly, because no one is going to generate enough energy to counteract gravity. Essentially you'd need a jet-powered willpower.)
So may answer would generally be "no," but I don't like closing my mind to possibilities. The covenmate of a friend had multiple brain tumors show up on a scan. They thought three were benign, but were concerned one was malignant. The coven did a ritual for her good health before she went into surgery. Afterward, the doctor reported that the three benign tumors were removed without incident. The patient's husband inquired about the fourth tumor, and the doctor rather embarrassedly reported that they couldn't find the fourth tumor. It was simply absent from where the scan said it should be.
Of course, there could have been an error with the scan. I certainly don't take the story as proof. I do take the story as a possibility.