It is known that during aging, the functionality of the immune system declines. As a result, elderly persons are more prone to develop cancers and are more susceptible to infection with viral, bacterial and parasitic pathogens. Examples are the increased susceptibility to infections with influenza viruses and Respiratory Syncytial Virus (RSV) that can cause severe respiratory disease. Another consequence of the reduced function of the immune system is that elderly persons do not respond to vaccination as well as younger individuals. Therefore, the influenza vaccine effectiveness in the elderly is suboptimal. A better understanding of the mechanisms of reduced responsiveness to infection or vaccination and immune control over persistent infections (also known as immunosenescence), like those caused by Varicella Zoster Virus (VZV) and of the defects of the elements that compose the immune system may aid the development of better intervention strategies and improved vaccines. Here our main interest is immunity to virus infections in individuals with different levels of immune-senescence.