Viruses may also emerge due to the establishment of human populations that are vulnerable to infection. For example, a virus may emerge following loss of cross-protective immunity, which may occur due to loss of a wild virus or termination of vaccination programmes. Well-developed countries also have higher proportions of aging citizens and obesity-related disease, thus meaning that their populations may be more immunosuppressed and therefore at risk of infection. Contrastingly, poorer nations may have immunocompromised populations due to malnutrition or chronic infection; these countries are also unlikely to have stable vaccination programmes. Additionally, changes in human demographics – for example, the birth and/or migration of immunologically naïve individuals – can lead to the development of a susceptible population that enables large-scale virus infection.