<p>Emerging (ED) and Reemerging Diseases (RED) is currently a worldwide concern, due to the unusual way in which they are presented, as a result of several factors related to changes in local ecosystems that alter the balance between the pathogens and their main hosts, in addition to the urbanization and changes in human behavior. A large number of pathogens (61% of the 1415 human pathogens identified) are from the zoonotic origin and 70% of them are originated from wild animals and can infect multiple species. This is perhaps due to the fact that humans have entered the natural habitats of different animal species in response to the social, political, economic and cultural demands of today’s globalization. That is because emerging and re-emerging pathogens have high biological flexibility that allows them to take advantage of the epidemiological opportunities in the environment. The variation in the epidemiological behavior of these diseases is associated with causes such as ecological, demographic, agricultural, trade, international travel, technology and industry, microbial adaptation, as well as modifications and failures in public health measures, and others. The objective of this review is to take a look at the main causes that have led to the emergence and re-emergence of diseases around the world.</p>