The transmission of communicative diseases has not changed over time since linked to biological and physiological attributes. A disease has a level of virulence and populations have levels of vulnerability. What has evolved with globalization is their epidemiology; the circumstances related to their spread among populations. The most significant factors include:
- Global travel. The speed and connectivity offered by air travel have become the most important factors in the global spread of diseases. The large number of people traveling for touristic and business purposes increases exponentially the risk of spreading flu-like diseases (viruses) rapidly and over long distances. The underlying connectivity, business and social interactions behind global air travel are associated with the initial epidemiology of an epidemic or a pandemic.
- Wars and conflicts. While the number and intensity of conflicts have substantially decreased in recent decades, they can be enduring and pervasive in several areas of the world. The related collapse of public infrastructures such as hospitals and public utilities (water and sewage systems) increases the vulnerability of the concerned populations. Further, conflicts are often associated with the internal and external displacements of populations (refugees), which may lead to the spread of diseases.
- Global trade. A less prevalent factor, but the risks associated with the unintended transport of pests or contaminated food, where bacteria are the vector. Otherwise, there are limited risks of the spread of diseases through trade.
- Migration. Large scale migration conveys the risk of migrants to transplant endemic diseases to new locations. This is particularly the case of migrants cluster within specific areas of their host countries.
- Poverty. While global poverty rates have plummeted, poverty is associated with malnutrition and unsanitary living conditions, making poor populations more susceptible.
- Medical practices. The large diffusion of antibiotics had the unintended consequence of enforcing a pathogenic natural selection, implying that viruses and microbes have greater resistance.