Emerging infections represent a concern especially when their increase is rapid and their mortality is high. They can be caused by previously undetected or unknown pathogens, by known agents affecting new geographical sites or new populations, or by re-emerging agents whose incidence of disease had significantly declined in the past. The main causes of emerging infectious disease (EID) spread include crowding; mobility (tourism, migration, asylum seekers, and refugees); centralization of food production and supply and new food vehicles; war, famine, and displacement; and change in vector distribution and susceptibility. Another important issue regarding EIDs is the source of pathogens, with 60.3% of cases from a non-human animal origin (zoonotic pathogens). If emerging viral infections represent a concern mainly in underdeveloped areas of the world, the emergence of antimicrobial resistance is a growing concern worldwide especially in developed and high-income countries, where misuse and overuse of antibiotics among humans and in the veterinary setting, including animal feeding, artificially created the selection of resistant strains. Future threats include not only diseases caused by viral agents, such as the case of Ebola virus disease periodical re-emergence, but also infections with antibiotic-resistant bacteria. It is estimated that in 2050, deaths due to antimicrobial-resistant infections will be higher than those due to cancer.