This session introduces the concept of ecology and evolution of infectious diseases. I will first talk about the view of host-pathogen interactions as part of the ecosystem. Second, the role of evolution in such a disease ecosystem, and its impact on disease control and prevention measures. Ecology is the study of interactions among organisms and their environment. A food web here shows an example of such interactions in an ecosystem. If the population of a prey, in this case, the rabbit, increases, the population of the predator, the fox, will start to increase because of more food. The predator population will drop after it depletes the prey population. Sometimes, animals have infectious diseases, which are often caused by parasitic pathogens. In microscopic view, the infectious diseases involve complex interactions of at least two species, the pathogen and the host. The pathogen lives in and obtains resources from a host, which can result in host death or development of immunity. It also relies on an individual host to transmit to other individuals, which is often a host density dependent process. This means more individuals get infected when the host population is denser, and less when the density is lower. When the pathogen spreads through and depletes the host population, the transmission will eventually slow down. Then the natural birth of host may restore the population, and subsequently the pathogen transmission will increase again when the host density becomes higher. This two-way regulation between the pathogen and host abundance forms the basic disease ecosystem and represents an important integral part of the larger ecosystem. In a disease ecosystem, some pathogens can infect more than one host. Other pathogens are transmitted from one host to the other through at least one species of vector. For example, malaria is a mosquito-borne disease caused by Plasmodium parasites and requires mosquitos as a vector for human and animal transmission. As a part of the larger ecosystem, environments also affect the emergence of infectious disease with the effect on the hosts, pathogens and vectors. For example, in some countries, a lower temperature and humidity facilitates human influenza transmissions. Level of rainfall also affects the occurrence of some water and vector-borne diseases. For example, malaria epidemics tend to occur during rainy seasons in the tropical countries because of the higher mosquito abundance. Cholera epidemics occurred in some poor areas during severe drought because of the water scarcity that results in poor hygiene; hence more people are exposed to the contaminated water. Such climate factors often drive an infectious disease into seasonal epidemics. Disease ecologists are often concerned about the burden to a host population by an infectious disease. Host extinction is perhaps the worst consequence caused by an infectious disease. However, in theory, a single-host pathogen with density dependent transmission seldom leads to host extinction because the transmission will naturally slow down when the host population drops. The disease will then fade out if all transmission chains are broken. In contrast, a multi-host pathogen could potentially cause the extinction of a host population. An example is the introduction of North American grey squirrels to England, which is resistant to the squirrel poxvirus. But that poxvirus is lethal to the native English red squirrels. Therefore, the poxvirus was maintained in the grey squirrels, and continuously transmitted to and infected the red squirrels even when the red squirrel population became very low. The poxvirus eventually wiped out the English red squirrels. All organisms, including pathogens, evolve. A new phenotype is sometime expressed from random mutations that occur in the reproductive process. Evolution takes place when nature tends to retain a new phenotype because of the advantage conferred to the organism. An example here shows how evolution works in a host-pathogen system. After a host recovers from the infection by a pathogen, adaptive immunity against this specific pathogen is often developed. Such immunity mainly recognizes the surface antigen of the pathogen – which is usually a protein encoded by the pathogen genome. When this recovered host is exposed to the same pathogen again, the adaptive immunity will protect the host from infection. However, pathogens are smart - they mutate. So, on the one hand, there is continuous natural selection of pathogen mutants that encode a varied antigen to escape from the pre-existing immunity. On the other hand, the host immune system continues to develop new immunity when the mutated pathogen invades the host. Such an arms race between pathogens and host immune system drives their continuous co-evolution, and shapes the patterns of disease emergence and re-emergence. An example of this is the human seasonal influenza. In addition to the annual influenza epidemics, which are probably driven by the climate, larger epidemics usually occur every several years. They were caused by the emergence of new antigenic variants of the influenza virus that were able to effectively escape from the pre-existing immunity in the human population. Vaccines developed for previous influenza variants often become useless to protect from these antigenic variants. While evolution of influenza is inevitable, one of the global efforts is to detect the new antigenic variants and produce the corresponding vaccine as soon as they emerge, so as to reduce the burden of these larger epidemics. So, viruses escape host immunity by their evolution. In a similar way, bacteria pathogens also develop drug resistance through their evolution. This has been highlighted by the recent expansion of multi-antibiotic-resistant bacteria. This becomes a global concern whether our evolution in drug development will win over the evolution of bacteria and viruses. In summary, pathogen-host interaction is an important integral part of the ecosystem. Better understanding of disease ecology and evolution help us to develop a more effective control and prevention measure against the disease.