Our group seeks to understand how viruses cause disease in humans. A better understanding of these mechanisms can lead to the development of vaccines to prevent new infections and anti-viral drugs to treat current infections. The lack of animal models that can be infected with human viruses and exhibit similar disease has seriously hampered these areas of research. In order to study viral infections of human cells in vivo, we use “humanized mice.” A humanized mouse is one in which human cells have been transplanted. We transplant mice with human hematopoietic stem cells; this leads to production of a variety of human blood cell types and development of a human immune system in the mouse. Antibody and cellular immune responses of human origin can then be generated in the mouse.