The extent is generally severe, with loss of 90% of circulating white cells (Okada et al., 2000), and affects at least CD4+ and CD8+ T cells and B cells (Okada et al., 2000; Wesley et al., 1978); other studies found that the proportions of different cell types remained unchanged during infection (Arneborn & Biberfeld, 1983; Griffin et al., 1986), suggesting that the initial leucopenia in measles also affects all subtypes, as we found for PPRV. The gene discussed is CD4; the disease is measles.