Is eczema caused by karma or is it viral in origin, or perhaps both, where karma might create susceptibility to the virus or aggravate its effects?

Your question is stated quite astutely from your knowledge of the interplay of karmic potentials and susceptibility to microbial attack having a karmic overlay many, many times, and that makes all the difference in who gets infections and who might resist them, even on a lifelong basis, through their resiliency. Eczema is typically the consequence of a viral attack on the body. When it concentrates in a localized area of skin, that can have assistance through prior karmic trauma to the surface of the body, as with wounding and things like burns, and so forth, causing painful lesions and disfigurement to compound the suffering. Such a history will make it more likely for a virus to take hold within the skin and reproduce a state of injury that is a match to the karmic potential energetically, and thus the two work hand in glove to cause trouble.