Multiple Sclerosis (MS): This is a leading cause of acute optic neuritis, especially in young adults. The presence of on and off fever could be indicative of an inflammatory process, which is consistent with MS. MS is known for its relapsing-remitting course, which might explain the intermittent fever.

Neuromyelitis Optica (NMO): Also known as Devic's disease, NMO can cause optic neuritis and may be associated with systemic symptoms like fever due to its autoimmune nature.
Systemic Lupus Erythematosus (SLE): SLE is a systemic autoimmune disease that can cause optic neuritis among its myriad manifestations. Fever is a common symptom in SLE due to its inflammatory nature.
Sarcoidosis: This condition can cause optic neuritis and is often associated with systemic symptoms including fever, due to granulomatous inflammation.

Syphilis: Especially in its secondary stage, syphilis can cause optic neuritis and is associated with fever among other systemic symptoms. Missing this diagnosis could lead to severe neurological complications if not treated promptly.
Lyme Disease: Caused by Borrelia burgdorferi, Lyme disease can lead to optic neuritis and is often accompanied by fever and other systemic symptoms. Early diagnosis is crucial to prevent long-term neurological sequelae.
CNS Lymphoma: Although rare, CNS lymphoma can present with optic neuritis and systemic symptoms like fever. Given its potential for severe outcomes, it's crucial not to miss this diagnosis.

Behçet's Disease: A form of vasculitis that can cause optic neuritis and is often associated with fever and other systemic symptoms. It's rare but should be considered in the differential, especially in patients from endemic areas.
Vogt-Koyanagi-Harada Disease: A rare autoimmune condition that can cause optic neuritis along with fever and other systemic symptoms. It's more common in certain ethnic groups and should be considered based on clinical presentation and patient background.