Epididymitis: This is the most likely diagnosis given the patient's symptoms of scrotal swelling, pain, and mild dysuria, especially following a brief illness with fever. The intermittent fever and the progression of symptoms over 6 days also support this diagnosis. Epididymitis is an inflammation of the epididymis, often caused by bacterial infection, which can be related to the patient's recent upper respiratory-like illness (ULI) if it was actually a urinary tract infection (UTI) or another infection that spread.

Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. Given the patient's recent illness and symptoms, orchitis is a plausible diagnosis, especially if the epididymitis has spread to involve the testicle.
Testicular Torsion: Although less likely given the gradual onset of symptoms, testicular torsion is a significant consideration, especially if the pain is severe and there's a history of similar episodes. However, the presence of fever and dysuria makes this less likely.
Hydrocele: An accumulation of fluid in the tunica vaginalis, a potential space around the testicle. While it can cause scrotal swelling, it typically does not cause significant pain or fever unless it becomes infected.

Testicular Torsion: Despite being mentioned as another likely diagnosis due to its severity and potential for testicular loss if not promptly treated, it's crucial to emphasize its importance again. The history of brief illness and fever might distract from this acute surgical emergency.
Fournier's Gangrene: A severe and potentially life-threatening condition characterized by necrotizing fasciitis of the genitalia. Although rare, the presence of fever, pain, and swelling, especially in the context of a recent illness, makes it essential not to miss this diagnosis.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that can cause scrotal swelling and pain among other symptoms. It's more common in children but can occur in adults, especially following an upper respiratory infection.
Spermatocele or Epididymal Cyst: These are benign conditions that can cause scrotal swelling but are less likely to cause significant pain or fever unless they become infected or rupture.
Malignancy: Testicular cancer can cause scrotal swelling and is a consideration, especially if the swelling is painless and there's a palpable mass. However, the acute onset of symptoms and presence of fever make this less likely.