Hepatitis A is a highly contagious liver infection caused by the hepatitis A virus. Despite the availability of an effective vaccine, the virus infects millions of people worldwide each year and remains a global public health problem, especially in underdeveloped countries. Unlike the hepatitis C virus, the hepatitis A virus does not establish a persistent infection. Yet, up to 20 percent of patients can relapse several weeks after virus growth and after symptoms have disappeared. "Mechanisms of immunity that protect against relapse, and why they occasionally fail, are unknown," said the study's lead author Christopher M. Walker, PhD, director of the Center for Vaccines and Immunity at The Research Institute at Nationwide Children's Hospital. Research has shown that white blood cells known as CD8+ killer T cells play a critical role in controlling hepatitis C and hepatitis B virus infections. These T cells act by killing infected liver cells, a process that damages the liver, but is necessary to effectively shut off production of new viruses.