Spinal infection of this type is rare but occurs in patients with the types of risk factors you mentioned. Any kind of immune suppression is a risk factor. Once the person has any kind of infection, it can spread to the joints, especially the hip, knee, or spine.
A previous history of infection somewhere else in the body is often a trigger for spinal infection. It's important to identify what kind of infection is present. Staph or strep infections are the most common.
Patients in good health can recover from infections of this type but the more typical response is for the infection to spread along the spine. Neurologic damage from pressure on the spinal cord can leave the person with permanent weakness and paralysis.
Surgery is advised to drain the infection. Antibiotics are an important follow-up. If at all possible, it might be good for your uncle to hear his physician talk about the potential complications. Treatment results are better with early intervention. Usually early diagnosis and treatment are needed to stabilize the spine.