4yo girl with 5lb weight loss during the last 2 months. no hx of illness, on no medications, immunizations up to date. 50th %ile for height, 25th %ile for weight. occasional crackles over right middle lung field. intradermal testing with ppd, tetanus, and antigens for candida and trichophyton is nonreactive at 72 hours. wbc count is 5100. nucleic acid hybridization testing of gastric aspirates shows tb. most likely explanation?