Plasmodium vivax is the most prevalent of the species of malaria and causes severe disease, but unlike P. falciparum it is rarely fatal. In 12 April Nature Hernando del Portillo and colleagues of the Universidade de Sao Paulo, Brazil suggest a possible explanation for the difference in virulence between the two parasites.

Sequence analysis of a 155 Kb yeast artificial chromosome clone from a P. vivax genomic library revealed a multigene family that is unique to P. vivax (Nature 2001, 410:839-842). Southern blotting showed that the genes — designated vir (P. vivax variant genes) — are present in 600-1,000 copies on possibly all 14 P. vivax chromosomes. Analysis of sera isolated from P. vivax patients indicated that each patient was infected with a parasite expressing a different VIR protein variant. This suggests that P. vivax is able to establish chronic infections by varying its appearance to the...