Context:
Q: How's everything going on with you and your family?
A: We are still struggling with long Haulers but some of the symptoms are improving. It's been 3 years now. We are just coming out of a bad spell of it.
A 'spell' is an unspecified period of time that usually represents something unpleasant. For example, a stay in hospital is called a spell, as is a period of time in prison. In your example, it is a period of illness - a particularly bad one.