Haemophilus ducreyi is a fastidious gram-negative coccobacillus bacteria, which causes the sexually transmitted disease chancroid, a major cause of genital ulceration in developing countries characterized by painful sores on the genitalia. Chancroid starts as an erythematous papular lesion that breaks down into a painful bleeding ulcer with a necrotic base and ragged edge. H. ducreyi can be cultured on chocolate agar. It is best treated with a macrolide like azithromycin and a third-generation cephalosporin like ceftriaxone. H. ducreyi gram stain appears as "school of fish."