Zoonotic spillover, which is the transmission of a pathogen from a vertebrate animal to a human, presents a global public health burden but is a poorly understood phenomenon. Zoonotic spillover requires several factors to align, including the ecological, epidemiological and behavioural determinants of pathogen exposure, and the within-human factors that affect susceptibility to infection. In this Opinion article, we propose a synthetic framework for animal-to-human transmission that integrates the relevant mechanisms. This framework reveals that all zoonotic pathogens must overcome a hierarchical series of barriers to cause spillover infections in humans. Understanding how these barriers are functionally and quantitatively linked, and how they interact in space and time, will substantially improve our ability to predict or prevent spillover events. This work provides a foundation for transdisciplinary investigation of spillover and synthetic theory on zoonotic transmission.

Most pandemics-eg, HIV/AIDS, severe acute respiratory syndrome, pandemic influenzaoriginate in animals, are caused by viruses, and are driven to emerge by ecological, behavioural, or socioeconomic changes. Despite their substantial effects on global public health and growing understanding of the process by which they emerge, no pandemic has been predicted before infecting human beings. We review what is known about the pathogens that emerge, the hosts that they originate in, and the factors that drive their emergence. We discuss challenges to their control and new efforts to predict pandemics, target surveillance to the most crucial interfaces, and identify prevention strategies. New mathematical modelling, diagnostic, communications, and informatics technologies can identify and report hitherto unknown microbes in other species, and thus new risk assessment approaches are needed to identify microbes most likely to cause human disease. We lay out a series of research and surveillance opportunities and goals that could help to overcome these challenges and move the global pandemic strategy from response to pre-emption.

SUMMARY Host range is a viral property reflecting natural hosts that are infected either as part of a principal transmission cycle or, less commonly, as “spillover” infections into alternative hosts. Rarely, viruses gain the ability to spread efficiently within a new host that was not previously exposed or susceptible. These transfers involve either increased exposure or the acquisition of variations that allow them to overcome barriers to infection of the new hosts. In these cases, devastating outbreaks can result. Steps involved in transfers of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host. Here we review what is known about host switching leading to viral emergence from known examples, considering the evolutionary mechanisms, virus-host interactions, host range barriers to infection, and processes that allow efficient host-to-host transmission in the new host population.

Canine parvovirus (CPV) is an emerging DNA virus that was first observed to cause disease in canines in 1978 and has since become a ubiquitous pathogen worldwide. CPV emerged from feline panleukopenia parvovirus (FPLV) or a closely related virus, differing at several key amino acid residues. Here we characterize the evolutionary processes underlying the emergence of CPV. Although FPLV has remained an endemic infection in its host populations, we show that, since the 1970s, the newly emerged CPV has undergone an epidemic-like pattern of logistic͞exponential growth, effectively doubling its population size every few years. This rapid population growth was associated with a lineage of CPV that acquired a broader host range and greater infectivity. Recombination played no role in the emergence of CPV. Rather, any preexisting variation in the donor species and the subsequent rapid adaptation of the virus to canines were likely dependent on a high rate of mutation and the positive selection of mutations in the major capsid gene. Strikingly, although these single-stranded viruses have a DNA genome and use cellular replication machinery, their rate of nucleotide substitution is closer to that of RNA viruses than to that of double-stranded DNA viruses.adaptation ͉ emergence ͉ mutation rate ͉ phylogeny ͉ natural selection P arvoviruses (family Parvoviridae) are small eukaryotic DNA viruses that infect a variety of animal species, including humans. Canine parvovirus (CPV), feline panleukopenia virus (FPLV), and a number of viruses similar to FPLV, such as blue fox parvovirus, the raccoon parvoviruses, and mink enteritis virus (MEV), are all host-range variants of the carnivore parvovirus subgroup (1, 2). Both disease and pathology differ depending on the age of the infected animal, because the viruses replicate only in cells in the S phase of the cell cycle. In neonatal animals, the virus replicates in a large number of tissues, and FPLV often causes cerebellar hypoplasia, whereas CPV causes myocarditis. In older animals, viral replication is limited to lymphoid and small intestinal cells, causing temporary panleukopenia or lymphopenia (3). FPLV is thought to have been endemic in felines since before the beginning of the 20th century (4). In contrast, it was not until 1978 that CPV (the first known strain was designated CPV2) was observed in canines, having emerged from FPLV or one of the closely related carnivore parvoviruses. Although CPV2 infected feline cells in culture, it did not infect cats. CPV2 was later replaced by a new lineage, designated CPV2a, which, along with its variants, can infect both dogs and cats (5, 6). There is also evidence that CPV2a is more effective at infecting canine cells than is CPV2 (7).Although epidemiological studies of emerging viruses are commonplace, the evolutionary processes associated with crossspecies virus transfer are poorly understood. Under some models, adaptation to the new host species is of fundamental importance, elevating the reproductive rate of the virus (R 0 ) above the...