Pathogens that can be transmitted between di¡erent host species are of fundamental interest and importance from public health, conservation and economic perspectives, yet systematic quanti¢cation of these pathogens is lacking. Here, pathogen characteristics, host range and risk factors determining disease emergence were analysed by constructing a database of disease-causing pathogens of humans and domestic mammals. The database consisted of 1415 pathogens causing disease in humans, 616 in livestock and 374 in domestic carnivores. Multihost pathogens were very prevalent among human pathogens (61.6%) and even more so among domestic mammal pathogens (livestock 77.3%, carnivores 90.0%). Pathogens able to infect human, domestic and wildlife hosts contained a similar proportion of diseasecausing pathogens for all three host groups. One hundred and ninety-six pathogens were associated with emerging diseases, 175 in humans, 29 in livestock and 12 in domestic carnivores. Across all these groups, helminths and fungi were relatively unlikely to emerge whereas viruses, particularly RNA viruses, were highly likely to emerge. The ability of a pathogen to infect multiple hosts, particularly hosts in other taxonomic orders or wildlife, were also risk factors for emergence in human and livestock pathogens. There is clearly a need to understand the dynamics of infectious diseases in complex multihost communities in order to mitigate disease threats to public health, livestock economies and wildlife.

Many infectious agents, especially those that cause emerging diseases, infect more than one host species. Managing reservoirs of multihost pathogens often plays a crucial role in effective disease control. However, reservoirs remain variously and loosely defined. We propose that reservoirs can only be understood with reference to defined target populations. Therefore, we define a reservoir as one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population. Existence of a reservoir is confirmed when infection within the target population cannot be sustained after all transmission between target and nontarget populations has been eliminated. When disease can be controlled solely by interventions within target populations, little knowledge of potentially complex reservoir infection dynamics is necessary for effective control. We discuss the practical value of different approaches that may be used to identify reservoirs in the field.

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Summary 1.We investigated the effects of three types of host on the persistence of a tick-borne virus, using the grouse-hare-deer-tick-louping ill virus system of upland Britain. Each host differed in its interaction with the vector and pathogen. Grouse amplify virus only, deer amplify vector only and hares amplify both. Grouse alone suffer high virusinduced mortality.2. An analytical model of the system was parameterized using empirical data from two wild populations with different community structures. By changing relative host densities we examined the conditions under which the virus would persist and considered the possibility of parasite-mediated competition between hosts. 3. Although deer alone and grouse alone were unable to maintain louping ill virus, a deer-grouse community usually allowed virus persistence because grouse transmitted virus while deer maintained the tick population. Since virus reduces grouse populations this is a type of apparent competition, and is unusual because deer do not amplify the virus. 4. At very high deer densities, the opposite effect could occur, whereby virus died out because of 'wasted' infected tick bites on deer, that do not transmit virus (the dilution effect). 5. In a hare-grouse two-host system virus usually persisted because hares amplified both the vector and virus (through non-viraemic transmission). Thus, apparent competition may occur between mountain hares and grouse. 6. The addition of a third host type increased the likelihood of disease persistence. Hares added to the deer-grouse system rendered the dilution effect unlikely because of additional virus amplifiers. Deer added to the hare-grouse system meant virus almost always persisted because they amplified the vector.

Juvenile mortality in cheetahs was found to be extremely high compared to other large mammals, with approximately 72.2% of litters dying before they emerged from the lair at eight weeks of age. An average of 83.3% of cubs alive at emergence died by adolescence at 14 months of age, thus cheetah cubs were estimated to have only a 4.8% chance of reaching independence at birth. The instantaneous rate of mortality was highest immediately after cubs emerged from the lair. Before emergence, lion predation was the major source of this mortality, although some cubs died from starvation after they were abandoned by their mothers, or as a result of grass fires and inclement weather. After emergence, predation again accounted for virtually all cub mortality, with lions and spotted hyaenas taking approximately the same proportion of cubs. Overall predation accounted for 73.2% of cheetah cub deaths in this study, with 78.2% of these being killed by lions. The extent of maternal care, in the form of vigilance and antipredator behaviour, mirrored cub susceptibility to mortality and, in the case of vigilance, possibly also starvation. The probability of a cheetah mother responding aggressively to a predator was found also to depend on the species of predator. This study highlights the importance of the influence of juvenile mortality on patterns of parental care.