Experimental design for quantification of Mycobacterium avium subsp. paratuberculosis among group-housed calves. One of 7 experimental pens represented, containing 2 inoculated (IN) calves (grey circles) and 2 contact exposed (CE) calves (white circles). a) 2 newborn calves entered the barn and were housed in a clean environment for 2 weeks; b) IN calves were inoculated with 5 × 108 CFU’s of MAP at 2 weeks of age; c) 2 weeks of IN housing to allow pass-through of inoculum. During this time, 2 newborn calves entered the barn and were designated as CE and housed apart from IN calves; d) first day of group housing. IN and CE calves were co-mingled in a clean pen and remained group-housed for 3 months; e) IN calves removed for necropsy, CE individually housed for additional 3 months.