The most important factors related to the occurrence of PCP may be classified into three groups: 1) host factors (e.g. CD4+ T cell counts [8]); 2) microorganism factors (e.g. Pneumocystis genotypes and virulence factors [9]); and 3) environmental factors (e.g. climatological factors and air pollutants [10,11]). The gene discussed is CD4; the disease is pneumocystosis.