Viral Infection: Given the symptoms of fever, generalized joint pain, and the CBC results showing a mild increase in WBC with elevated monocytes, a viral infection such as influenza or a viral syndrome is a plausible diagnosis. The elevated monocyte count often indicates a viral or chronic infection.

Bacterial Infection: Although the neutrophil count is not significantly elevated, bacterial infections such as pneumonia or a urinary tract infection could still be considered, especially if the patient has other symptoms like cough, dysuria, or flank pain.
Rheumatologic Disease: Conditions like rheumatoid arthritis or lupus could present with joint pain and fever, although the acute onset and specific CBC findings might make these less likely without additional symptoms or history.
Reactive Arthritis: This condition, often following a bacterial infection, could explain the joint pain and fever, especially if the patient has recently had an infection like gastroenteritis or urethritis.

Septic Arthritis: Although less likely given the generalized nature of the joint pain, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. The presence of fever and joint pain mandates consideration of this condition.
Endocarditis: Infective endocarditis can present with fever, joint pain (due to embolic phenomena), and elevated WBC. It's a critical diagnosis not to miss due to its high morbidity and mortality.
Lymphoma or Leukemia: While less common, these conditions can present with fever, joint pain, and abnormal CBC results, including elevated monocytes. They are important to consider, especially if the patient has other systemic symptoms like weight loss or night sweats.

Adult-Onset Still's Disease: A rare condition characterized by high fevers, joint pain, and a distinctive salmon-colored rash, along with elevated WBC and ferritin levels.
Hematologic Disorders: Other rare hematologic conditions, such as myeloproliferative neoplasms or myelodysplastic syndromes, could potentially explain the patient's symptoms and CBC findings, although they would be less likely without additional specific findings or history.
Sarcoidosis: This condition can cause fever, joint pain, and elevated WBC, but it would typically be accompanied by other symptoms like lung findings or skin lesions.