Department of Microbiology and Immunology, Rega Institute, KU Leuven, Leuven, Belgium.|Department of Microbiology and Immunology, Rega Institute, KU Leuven, Leuven, Belgium.|Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, UK.|Department of Human Genetics, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.