You probably recognize this scenario, there you are, minding you're own business, walking down the street when someone comes up behind you and sneezes. You're polite, kind and respond with a, bless you, and you think little more about. Later that day, you start to feel queezy, joints start to ache, within 24 hours you're in your bed suffering from a throbbing head and you feel like death warmed up. Your mind goes back to that ocassion and the person that sneezed on you. And while you there, and you're in misery, you start to ask yourself some questions. Where on earth did this infection come from? Surely you've had every infection going, where are these new ones coming from? And you want to blame someone, just anyone for this dreadful way you feel so, who's to blame? Well, well then 70% of newly emerging infections are derived from wildlife. And a proportionally large number of these are like the RNA viruses. These are derived from that turning infections of wildlife, that is spilledover into humans. Indeed humans are infected with about 1400 infections. And we know that 61% of human infections were actually derived from wildlife. This type of infection, that is one that's transmitted from a non-human animal to a human, is what we call a zoonotic disease, and they include all the historically important infections of humans. For example, bubonic plague. Spread from the Far East into Europe, and in 1347, killed a third of the human population. In some areas, as many as 60% of the human population died. And it took countries like Italy, more than 400 years, just to recover their population size. That is truly a devastating impact. Where did infection come from? Well, the zoonotic disease that came from rats and transmitted through the bite of a rat flea to the humans. Another one, smallpox, has had a profound effect on the history of humans. It decimated the Ethiopian soldiers in the Elephant War in Mecca in 568 AD. In 1502, it was introduced into the western world where it swept through the Amerindian tribes and resulted in the collapse of both the Aztec and the Incan empires. In 1738, it killed half the Cherokee nation. Where did it come from? Well, we believe it probably spilledover from rodents 10,000 years ago. The 1918 influenza pandemic was another one. Caused by a particularly virulent influenza strain that infected 500 million people and killed in the order of 100 million people. Where did it come from? It was a zoonotic disease that came from birds, then infected both humans and pigs. HIV 1 is another one, currently infects more than 30 million people in the world. And causes more than a million deaths each year. It was clinically first observed in 1981, but where did it come from? It was a zoonotic disease that spilledover from chimpanzees to humans. And indeed has done so in at least three occasions. In most instances, a zoonotic disease just spillsover from the reservoir into the human population. It may infect one or two people. But then, with no one transmission just fades out and left as an undiagnosed or misdiagnosed infection. A few infections have become well-established. Well established human infections. HIV is a good example and is a special, because it has persisted within the human population. And will clearly remain with us for many, many years to come. Dengue fever is another zoonotic disease. That has circulated naturally among primates, and then transmitted to humans by the aedes mosquitoes. But has now become well established within the human populations, in Southeast Asia and persisted as a human infection. Now with these zoonotic diseases we're looking at a spillover process. A process that involves a series of steps. First, the pathogen must move from the reservoir animal host to the human host. Then once it reaches the human host it must successfully infect the host and reach the right tissue. Multiply, and leave the host so the host sheds the pathogen and is capable of transmitting. Of course, the host must then be able to spread the infective stages, in the manner that will results infection of a [UNKNOWN] another susceptible host. If the pathogen is going to persist for at least a period of time, in the host population, then, of course, the basic reproductive number R naught must be greater than one. And the infection will need mechanisms to ensure survival. The type of dy, spillover dynamics we observe, can really be put into three different categories. The first category is where R naught, R naught is close to zero. In the human population, there is no one with transmission. In this instance, the infection simply spills over from reservoir to human, but there's no one with transmission at all. So, this includes infections where there's no outward transmission, but the infection still generates disease symptoms in the humans. This group includes infections like lyme disease, where there is quite simply no onward transmission because humans don't host many ticks that survive to infect anyone else. Another one is rabies, where anyone showing disease symptoms will be quickly treated or quarantined, and so there's no onward transmission. The force of the infection determines what proportion of humans become infected. It's not that they're becoming infected from other humans. The second category is where R naught is close to one and we get stuttering chains of transmission amongst humans. In this instance, there is someone with transmission, but these transmission chains get broken, so there are just a few human cases before it eventually fights out. A good example of this is monkeypox virus in the Congo basin. Here, children have become infected when they catch and eat Gambian rats. Infected children may infect a small number of other children. But the chains of transmission are short. And so the infection disappears locally, until the next spillover event occurs, when another child becomes infected from another rat. These are worrying infections. There is onward transmission and if that pathogen could adapt to the human case, they could become serious human infections. In the instance protection against monkeypox provides to the human population throughout smallpox vaccination. But now, smallpox has been eradicated. And vaccination has stopped. Monkeypox prevalence is increasing, and there's some concern that monkeypox could adapt and re-emerge as a replacement for smallpox. The third category, is where R naught is substantially greater than one. And there is sustained onward transmission. This is where spillover occurs. And like any other successful infection, the pathogenic [INAUDIBLE] onward transmission from one host to the next. And this can result in an epidemic outbreak. Of course, this does not mean that the infection is going to persist in the human population. It may simply burn through a series of hosts, and then just fade away. One example of this is Ebola virus, where it spills over from some wild host species into the local popula, population. And spreads rapidly, killing a high proportion of people in the local community before effectively burning out and disappearing. It's simply too virulant to generate a sustained and persistent infection. These new infections produce a great threat to humanity. Our transport systems. A large number of global connections, mean that any new infection could spread globally, and have a massive impact on the human population. We really need to know when the next HIV, the next West Nile, or SARS outbreak is going to occur. How shall we prepare ourselves, for identifying the threat? And, how should we respond to protect people globally. I was part of a team that undertook a review of what we know, and don't know about these emerging zoonotic diseases. And it became apparent that we know. Is based on really, acute, direct, viral and symptomatic infections. Infections just like influenza. In contrast, we know very, very little about the chronic diseases. The vector-borne ones. The asymptomatic ones, and the protozoan infections. There's a tendency for the research to focus either on the reservoir or the human population. But very rarely to connect the two and understand these spin out of events of what's taking place. Indeed only 2% of studies that modeled infections of symbiotic diseases included the transmission mechanism spillover. This is clearly an area of research in need of much deeper study if we're going to understand and perhaps predict where these infections arise.
