MD incidence and tumor formation in chickens infected with the rRB-1B EGFP22 virus. A. Cumulative incidence of MD in birds inoculated with rRB-1B EGFP22 or rRB-1B viruses (n = 12), and contact naive birds of the same age (n = 9 to 11) housed together (Experiment 1). Viruses (1000 pfu) were inoculated intramuscularly into groups of 1 week-old, B13/B13 White Leghorn chickens. MD incidence was determined by identification of gross lesions at the necropsy. In both groups, at the end of the experiment, a few chickens showing no clinical signs had tumors. The cumulative incidence of the disease is expressed as a percentage. B. Lymphoma incidence in inoculated birds of each group, according to the number of organs presenting macroscopic MD lesions in each bird.