Viral Arthritis: This is a common cause of generalized joint pain and fever in adults. Viral infections such as parvovirus B19, hepatitis B, and HIV can cause arthritis. The absence of abdominal pain and the presence of fever and joint pain make this a plausible diagnosis.

Rheumatoid Arthritis (RA): Although RA can present at any age, it typically starts in the 30s to 60s. The symptoms of generalized joint pain and fever could be indicative of an RA flare, especially if the patient has a history of the disease.
Lyme Disease: Caused by the bacterium Borrelia burgdorferi, Lyme disease can present with fever, joint pain, and other systemic symptoms. The diagnosis would depend on exposure history and geographic location.
Reactive Arthritis: This form of arthritis occurs as a reaction to an infection elsewhere in the body, often in the gastrointestinal tract or the urethra. The absence of abdominal pain does not rule out reactive arthritis, as the triggering infection might not cause significant gastrointestinal symptoms.

Septic Arthritis: Although the patient does not have localized joint pain or significant swelling, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with systemic symptoms like fever and can quickly lead to joint destruction if not addressed.
Endocarditis: Infective endocarditis can cause fever, joint pain (due to septic emboli or immune complex deposition), and other systemic symptoms. It is crucial to consider this diagnosis, especially if the patient has a history of heart disease or intravenous drug use.
Osteomyelitis: Bone infection can cause generalized pain, fever, and other systemic symptoms. While it might seem less likely given the generalized nature of the joint pain, osteomyelitis can present in various ways and is important not to miss due to its potential for serious complications.

Adult-Onset Still's Disease (AOSD): A rare inflammatory disorder that can cause high fevers, joint pain, and rash. It is a diagnosis of exclusion but should be considered in patients with systemic inflammation and joint pain without a clear cause.
Sarcoidosis: A condition characterized by the growth of granulomas (inflammatory cells) in various parts of the body, which can cause fever, joint pain, and other symptoms. It is less common but can present with a wide range of symptoms, making it a consideration in complex cases.
Hemochromatosis: A genetic disorder causing the body to absorb too much iron from food, leading to iron overload. It can cause joint pain (similar to osteoarthritis), fever, and other systemic symptoms, although it is less likely to present acutely with fever and generalized joint pain.