Vaccines have led to many of the world’s greatest public health triumphs, but many deadly viruses, such as HIV, still elude the best efforts of scientists to develop effective vaccines against them. An improved understanding of how the immune system operates during a viral infection is critical to designing successful anti-virus vaccines. Scientists from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), have added an important dimension to this knowledge.