Birth, death, old age and diseases are the four inevitable miseries of material life. But where did our diseases come from? The epidemiologic triangle illustrates the relationships among the three essential factors that are involved in the disease development. For a disease or injury to occur, the basic elements of disease causation must be present. When this outside agent meets a vulnerable host in an environment that allows the agent and host to interact, a disease or injury may be established. Although this model applies to either biological, chemical or physical agents, today we would like to focus on infectious disease-causing pathogens only. Infectious diseases are diseases caused by microbes that spread. These microbes include viruses, bacteria, fungi, protozoa, helminths and prions. and diseases caused by them can be classified into two categories based on their natural reservoirs. An infectious disease with a natural reservoir as an animal host is called zoonosis, while those with human as natural reservoir are called anthroponoses or reverse zoonoses. In a systematic review of over 1,400 pathogens known to infect humans, more than 60 percent were zoonotic agents, and their invasion into the human population are called ‘disease emergence’. Notably, the activity of the emerging infectious diseases dramatically increased in the past decades, and emerging infections are now accounting for at least 12 percent of all human pathogens. In this map, you will find emerging infectious diseases were caused by newly identified species such as SARS, AIDS or Nipah viruses; or strains that may have evolved from a known pathogen such as the H5N1 influenza virus; or pathogens that spread to a new population or geographic area, such as the West Nile virus; or that emerged in regions undergoing ecological transformation, such as the Lyme disease agents; or, be re-emerging infections and drug resistant agents. Among all these emerging pathogens, most of them were of zoonotic origins and were viruses, especially RNA virus species. RNA viruses have small genomes and can replicate rapidly, but inaccurately. This is attributed to their error-prone RNA polymerase, which does not have a proofreading function. When large population of non-identical progeny was produced within a host, selection and divergence could occur to facilitate the generation of novel variants with better fitness to the environment. As the ecosphere surrounding human beings are different species of domestic and wild animals, zoonotic disease agents carried by these animal hosts may be directly or indirectly transmitted to the human population via airborne, water-borne, vector-borne or direct contact transmissions. In particular, the free range poultry, livestock and pets may serve as the intermediate hosts for bringing the pathogens from the wildlife to our daily life. Modern animal farming, collecting and distributing centers, such as live poultry markets, and temporary holding centers, agricultural exhibitions, wildlife rescue and trading systems, also play an important role in the generation of novel viruses and their inter-species transmission to the human populations. Take the emergence of the Asian H5N1 and H7N9 influenza viruses as an example. These viruses originated from wild ducks, amplified and re-assorted in the domestic ducks and subsequently chickens, then brought to the live poultry market and transmitted to humans. Occasionally, these enzootic viruses could also find a way to transmit back to the wild migratory birds, and disseminated to other countries or even continents. Live animal trading and usage of illegal vaccines also contributed to the spread of viruses across regions. Bats are considered as the natural reservoir hosts of many RNA viruses. Although rarely reported, direct attacks by bats infected with the Lyssaviruses have caused a few human fatal cases in Australia. Virus transmissions from bats to the domestic animals and then further to the human beings are more commonly seen in the virus emergence. Nipah virus is a paramyxovirus first identified in 1999, when it caused an outbreak on pig farms in Malaysia, and resulted in hundreds of human infections and a fatality rate of around 40 percent. Eight more outbreaks of Nipah virus have occurred since then, all within Bangladesh and neighboring parts of India. Human-to-human transmission was observed, and fruit bats were recognized as the primary reservoir, and pigs as the intermediate hosts. Overlapping habitats and spillage of urine, faeces and partially eaten fruits from bats to pigs or humans were thought to be the root of virus transmission. In Australia, a similar paramyxovirus, Hendra virus, caused a total of 50 outbreaks, all involving horses and some with human fatalities. This virus also originated from the flying fox, which congregated in Queensland during the winter seasons. Other examples include the SARS and MERS coronaviruses that have emerged from civet cats and camels, and have caused serious public health problems in the world. Taken together, the emergence of many emerging infectious human diseases are associated with inter-species transmission and subsequent adaptations in the new hosts. Surveillance of the disease agents at the human and animal interface could provide early warnings and shed light on the genesis and development of a pathogen. Robust risk assessment is a core part of public health response. And communications, transparency and sharing of information are essential for the early intervention and evidence-based control measures.