B.M., a 20-years-old female was referred to our Rheumatologic Clinic in March 2009 for an overt arthritis involving the third proximal interphalangeal (PIP) joint of the left hand and the first metacarpophalangeal (MCP) joint of the right hand. Beside arthritis, which appeared six months before, the patient presented fatigue, morning joint stiffness for about three hours, and more recently pain also in the wrists and in the other MCP and PIP joints. At the physical examination, both the knees and wrists were swollen, together with the firstly involved joints. Laboratory tests showed slight increase of erythrocyte sedimentation rate (24 mmh) and c-reactive protein 1.4 mg/dL, presence of serum rheumatoid factor (182 UI/mL; n.v. < 14), anticitrullinated peptide antibodies (251.6 UI/mL; n.v. < 25), while antinuclear antibodies were negative. Firstly, in the hypothesis of reactive arthritis because of a transient genital infection by Ureaplasma urealyticum, prednisone 12.5 mg/day tapered to 5 mg was administered for about three months; but, arthritis did not show any significant improvement. Radiograms of the hands evidenced typical marginal erosion at the basis of the phalanx of the third PIP joint. On these basis, a diagnosis of early RA was made, and a treatment with methotrexate 10 mg/week plus prednisone 5 mg/day and diclofenac 150 mg/day was started.
In the following 3 months, the patient did not experience a significant improvement of her symptoms, so she continued to take diclofenac every day. Similarly, leflunomide 20 mg/day in association with diclofenac and steroids did not show satisfying results. After a negative screening for hepatitis B and C virus and Mycobacterium tuberculosis, a treatment with infliximab 3 mg/kg was decided, considering the activity and the severity of arthritis. The first infusion was performed on 15th October 2009, without steroid or antihistaminic premedication; no adverse reactions were registered during the infusion and in the following week. On 26th October, the patient presented a macular rash on the neck and the trunk (Figure 1), and painful lymphonodes behind the ears and on the neck; fever of 37.5 C and sore throat were also present.
Suspecting a viral infection, a serological evaluation for measles, EBV, Cytomegalovirus, and Parvovirus B19 was performed, using an immunoenzymatic reaction kit (Technogenetics-Bouty, Milan, Italy). EBV-VCA IgM resulted 2.01 (values > 1.10 were considered positive), EBV-VCA IgG 121 (values > 11.5 were considered positive), along with serum EBV-EBNA IgG (memory antibodies). The IgM antibodies directed against the other investigated viruses were negative. 
All the above symptoms disappeared in five days spontaneously and did not reappear up to date. In the same time, arthritis dramatically improved after the first infusion of infliximab, with apparent resolution of synovitis of the wrists, knees, and hands. The second infliximab infusion was done one month later, after the complete resolution of the EBV reactivation symptoms.