Cytomegalovirus (CMV) is one of the reasons our immune systems decay with aging: too many immune cells become specialized to deal with CMV, leaving too few to deal with everything else. New research "explains how a virus that has already infected up to 80 percent of the American population can repeatedly re-infect individuals despite the presence of a strong and long-lasting immune response. The research involves cytomegalovirus (CMV), which infects 50 percent to 80 percent of the U.S. population before age 40. ... For most people, CMV infection goes undetected and they do not become seriously ill. ... When most viruses infect a host, the immune system remembers the disease and protects against re-infection. This is the case with smallpox, seasonal strains of flu and several other viruses. This immune system reaction is also the reason why vaccines made with weakened or dead viruses work against these pathogens. In the case of CMV, the body's immune system is continuously stimulated by ongoing, low-level persistent infection, but yet CMV is still able to re-infect. This research explains how CMV is able to overcome this immune response so that re-infection occurs. ... The results of this study primarily illustrate the significant barriers to creating a vaccine that will prevent CMV infection." But a vaccine won't do much for people already burdened by an CMV-focused immune system. What we want is a way to use targeted cell killing strategies to destroy CMV-related immune cells and free up space for more useful immune cells.