Sort of viruses - an autoimmune reaction to a protein produced by virus fragments which are part of human DNA. But really, the problem is the autoimmunity. That said, autoimmune disease can be triggered by proper viruses. So autoimmunity might be the underlying problem, but it could have been a virus that triggered the autoimmunity. Then dysregulated immune system possibly results, and abnormal reactions to infections? There's no detailed info about how other symptoms are caused.