Syphilis

As it turns out, there's a specific type of "tertiary syphilis" called Late Neurosyphilis which develops a few years after initial infection and can affect the central nervous system. Symptoms include seizures, "lightening pain the lower extremities," poor balance (so it makes sense that Act V Alva can't remain standing for long), and dementia (so it makes sense that he's a little loopy).

A person in this stage of syphilis isn't contagious. So Alwa and Lulu could still be doin' it and Lulu could remain healthy.