In May 2008, a 38-year-old man with a human immunodeficiency virus (HIV) infection reported swelling and tenderness in the perianal region. During defecation, a brown liquid was released spontaneously from the swollen tissue. Physical examination confirmed a perianal abscess but was otherwise without pathological findings. The laboratory examination revealed erythrocytes 4.09×109/L (normal 4.5-5.5), mean corpuscular volume 102.0 fL (normal 83-93), leukocytes 19.2×106/L (normal 4-9), fibrinogen 8.0...