It is known from studies in pigs that infections with virulent strains of Mhyop are associated with high levels of interleukin-β and tumor necrosis factor in bronchial lavage fluids [61,72], and that these cytokines have a nonspecific mitogenic effect on lymphocytes, resulting in BALT hyperplasia [61,72,81,82]. The gene discussed is TNF; the disease is infection.