Emerging and re-emerging infectious diseases are increasing worldwide. Over the last thirty years, more than 30 new human pathogens have been discovered, including Ebola, Nipah, Hendra, Zika, Severe acute respiratory syndrome (SARS), Middle-East respiratory syndrome (MERS), Novel Coronavirus (COVID-19) and 75% of these are known to originate from animals. Viruses are potent infectious agents causing numerous life-threatening diseases in human beings and co-evolved with human evolution. Emerging viruses, especially the RNA viruses, have high potential to adapt rapidly to a new host and changing environments than other pathogens. Many factors related to host, enviroment and microorganisms that have been increasing in recent years bring new emerging and re-emerging infectious diseases to the agenda.