Since 1980, the world has been threatened by different waves of emerging disease epidemics. In the twenty-first century, these diseases have become an increasing global concern because of their health and economic impacts in both developed and resource-constrained countries. It is difficult to stop the occurrence of new pathogens in the future due to the interconnection among humans, animals, and the environment. However, it is possible to face a new disease or to reduce the risk of its spread by implementing better early warning systems and effective disease control and prevention, e.g., effective global surveillance, development of technology for better diagnostics, effective treatments, and vaccines, the global political will to respond to any threats and multidisciplinary collaboration involving all sectors in charge of good health maintenance. In this review, we generally describe some factors related to human activities and show how they can play a role in the transmission and spread of infectious diseases by using some diseases as examples. Additionally, we describe and discuss major factors that are facilitating the spread of the new pandemic known as COVID-19 worldwide.