Why are we forced to face the risk of pandemic? And there is a simple answer, by looking at the mechanism of emergence in an ecological point of view. This figure shows the schematic illustration of the mechanism by which a pandemic influenza emerges. In the daily life, in a human host, the influenza virus can be maintained and transmission is sustained. But not only in the human host, but in the wildlife. Especially in the wild waterfowls, such as mallards as shown here. The influenza virus is known to be maintained. And all different subtypes of influenza virus, that include those which are not seen in the human's population yet, are harbored by wild waterfowls. And there are continuous spillover of viruses from wild waterfowl through the population of a chicken or turkey as well as a swine, pigs. And in pigs, it is known that the co-infection of different influenza viruses can occur. And so for example, if the swine is infected with viruses from avian origin, and also from human origin, the recombination can occur. And that can sometimes create what we call the new reassorted virus, that could act as a novel influenza, pandemic strain in the human population. The same thing can be seen in the other emerging infectious diseases. And the easiest example is found in the case of the Ebola virus disease that has caused a huge epidemic from 2013 to 2015. Ebola virus is known to be maintained by the population of a fruit bat. And the Ebola virus transmission is maintained in that population. And the fruit bat is abundant from central to western African region. And sometimes, the spillover can occur to the other animal species, especially the primates, including, gorilla, chimpanzees, and human is one of the primates that can be infected. And in this 2013 to 2015 epidemic, the human hosts initially experienced a spillover of the virus followed by the transmission within the human population. So for that reason, the human population is continuously exposed to the spillover from the wildlife. And as long as the infectious disease is harbored in our wildlife population, especially that those called as reservoir host which can maintain the disease, we are forced to continuously face the risk of emergence of infectious diseases. So such diseases is referred to as Zoonosis. Zoonosis is defined as an infectious disease spreading from vertebrate animals to humans. And when we talk about the Zoonosis, the focal host that we are interested is referred to as the reservoir host. Which is defined as host species which can maintain pathogen or transmission in a population. And in many instances, as in the case of the wild waterfowl and the fruit bat, natural reservoir host in wildlife cannot be manually controlled. And for that reason, we are forced to accept the continuous increase- continuous spillover of viruses or bacteria coming from wildlife to human society. However, the question remains. I have told you that there is an increasing trend of the emerging infectious diseases, why has it increased? Why did zoonotic diseases increase over time? There are a number of explanations and the debates are under validation. But here I enumerated potential factors that have been frequently referred when we talk about the increasing trend of an emerging infectious disease. The first factor is an epidemiological transition in the North. Epidemiological transition is a transition in the cause of this. In industrialized countries, The infectious diseases are getting less and less common, and on the other hand, the deaths caused by cancer and cardiovascular diseases replaced the previous death caused of our infectious diseases. That has been almost a complete in the North whereas a transition is still underway in the South and the keeping that situation. There's an increased mobility using aircraft across countries. And the susceptible people from the North are exposed to infected individuals in the South. Moreover, there's an increasing trend in the access to wildlife. Increasing the direct exposure frequency and therefore- thereby exposing travelers at risk of infection with a novel infectious disease. The third factor is not general to all emerging infectious diseases. But is a case for eliminated or eradicated disease such as smallpox. Smallpox was eradicated in the 1980s. And in 1980s the routine vaccination was seized. And for that reason, those who are born after- in and after the 1980s were unvaccinated and in the future our population may be 100% susceptible to smallpox, leaving that population vulnerable to the smallpox epidemic which may be caused by a bioterrorist attack. The fourth point is the industrial revolution in the livestock animal factories. In feeding the livestock, there has been a technological improvement in growing and factoring the animals. But on the other hand, as a drawback, there have been the increased risk of the emergence of novel infectious diseases. However, we do not have to be too pessimistic about the emergence of the novel infectious diseases to the human society. Actually there are known barriers of the emergence before the wider spread of the virus in the human population. So this figure schematically illustrated the known barriers. So before the virus, the novel virus is introduced from wildlife to humans, there must be the interspecific transmission from animal to human, which is extremely rare. And even provided that the infection is established in humans, there's a B, the barrier B, which is a virus host interaction within an individual. Usually the animal virus is still not ready to be replicated within a human body and the growth of the virus is not substantial. And even provided that the infected individual experiences replicated increase of the viruses, the virus must be transmitted from one person to the other to maintain the epidemic. And the interspecific host to host contact may also be limited in the case of the novel emerging infectious diseases. And moreover, that has to persist in a population. In other words, without substantial transmission potential, the novel infectious disease cannot lead to a global pandemic immediately. Therefore, even provided that the novel infectious disease emerges continuously. The majority of the speed of events decline to extinction. So in summary, we have studied a number of factors as enumerated here. The pandemic is defined as a global epidemic. The many emerging infectious diseases are of zoonotic origin, maintained by wildlife animal species. Among the wildlife animal species, the reservoir host refers to the host species that can maintain infection in wildlife. And in many instances, we cannot manually control the reservoir host and the human society is forced to face the continuous risk of infectious diseases. Lastly, even provided that we have to face the risk of emerging infectious disease risks, the inter-specific and within-host barriers exists before the spillover leads to a global pandemic in human society. Thank you.
