Viral Meningitis: This diagnosis is the most likely due to the combination of symptoms such as headache, neck pain, photophobia, and altered level of consciousness (ALOC), which are classic for meningitis. The presence of subjective fevers and general malaise also supports an infectious process.

Bacterial Meningitis: Similar to viral meningitis, bacterial meningitis presents with headache, neck stiffness, fever, and photophobia. However, it tends to have a more severe and rapid onset. The presence of ALOC and vomiting could indicate increased intracranial pressure, which is more commonly associated with bacterial meningitis.
Encephalitis: This condition, which is an inflammation of the brain tissue, can present with headache, fever, confusion, and sometimes seizures. The ALOC and general malaise could be indicative of encephalitis, especially if there's a viral or autoimmune cause.
Subarachnoid Hemorrhage: Although less likely given the absence of a reported "thunderclap" headache, a subarachnoid hemorrhage could present with sudden onset of severe headache, vomiting, and altered mental status. Neck pain and photophobia could also be present due to blood in the subarachnoid space irritating the meninges.

Cerebral Venous Thrombosis: This condition can present with headache, vomiting, and altered mental status, especially if there's associated cerebral edema or infarction. It's crucial not to miss this diagnosis due to its high morbidity and mortality if not treated promptly.
Brain Abscess: Although rare, a brain abscess can present with similar symptoms, including headache, fever, and altered mental status. The presence of neck pain and photophobia could indicate meningeal irritation.
Meningoencephalitis due to Other Causes (e.g., Fungal, Parasitic): In immunocompromised patients, these causes can lead to similar presentations and are critical not to miss due to their high mortality rates if not treated appropriately.

West Nile Virus Meningoencephalitis: This viral infection can cause symptoms similar to those presented, including fever, headache, and altered mental status. It's more common in certain geographic areas and during specific times of the year.
Lymphocytic Choriomeningitis (LCMV): Caused by an arenavirus, LCMV can lead to meningitis or meningoencephalitis, presenting with fever, headache, and sometimes altered mental status. It's rare but can be severe, especially in immunocompromised individuals.
Rickettsial Infections: Certain rickettsial diseases, like Rocky Mountain spotted fever, can present with fever, headache, and altered mental status, although they are less common and typically associated with a rash and specific epidemiologic exposures.