A 35-year-old man presents with fever, fatigue and malaise, generalized arthralgia and myalgia, and a diffuse headache for 14 days. No other symptoms are present. He has not sought medical attention so far, instead self-medicating himself with acetaminophen (paracetamol). He returned from a week-long hiking trip in rural Massachusetts around three weeks ago. He does not recall being bitten by any insects. Three friends accompanied him; all are currently healthy. There is no contact history with people with similar symptoms. He has a single stable long-term sexual partner. His medical, surgical and family histories are unremarkable. He does not smoke and only drinks socially. His basic investigations are as follows:

WBC: 5,100/mm3 (4,600-11,00)

Neutrophils: 45%

Lymphocytes: 37%

Hb: 10.5 g/dL (11-18)

MCV: 90 fL (80-96)

MCH: 30 pg/cell (27-33)

Platelets: 196,000/mm3 (150,000-450,000)

ALT: 80 IU/L (7-45)

AST: 72 IU/L (8-48)

Total bilirubin: 1.5 mg/dL (0.2-1.2)

Direct bilirubin: 0.3 mg/dL (0.1-0.4)

Reticulocyte count: 8%

Serum LDH: 800 IU/L (105-333)

Direct antiglobulin test: negative

Indirect antiglobulin test: negative

Renal functions, chest x-rays, and a urinalysis are all normal.