In conclusion, this is the first complete documentation of pandemic influenza in mainland China, to our knowledge. The findings indicate that interventions focused on domestic travel, population density, and climatic factors could play a major role in mitigating the public health impact of future influenza pandemics. Replication, Neurotropism, and Pathogenicity of Avian Paramyxovirus Serotypes 1–9 in Chickens and Ducks Avian paramyxovirus (APMV) serotypes 1–9 have been isolated from many different avian species. APMV-1 (Newcastle disease virus) is the only well-characterized serotype, because of the high morbidity, mortality, and economic loss caused by highly virulent strains. Very little is known about the pathogenesis, replication, virulence, and tropism of the other APMV serotypes. Here, this was evaluated for prototypes strains of APMV serotypes 2–9 in cell culture and in chickens and ducks. In cell culture, only APMV-1, -3 and -5 induced syncytium formation. In chicken DF1 cells, APMV-3 replicated with an efficiency approaching that of APMV-1, while APMV-2 and -5 replicated to lower, intermediate titers and the others were much lower. Mean death time (MDT) assay in chicken eggs and intracerebral pathogenicity index (ICPI) test in 1-day-old SPF chicks demonstrated that APMV types 2–9 were avirulent. Evaluation of replication in primary neuronal cells in vitro as well as in the brains of 1-day-old chicks showed that, among types 2–9, only APMV-3 was neurotropic, although this virus was not neurovirulent. Following intranasal infection of 1-day-old and 2-week-old chickens, replication of APMV types 2–9 was mostly restricted to the respiratory tract, although APMV-3 was neuroinvasive and neurotropic (but not neurovirulent) and also was found in the spleen. Experimental intranasal infection of 3-week-old mallard ducks with the APMVs did not produce any clinical signs (even for APMV-1) and exhibited restricted viral replication of the APMVs (including APMV-1) to the upper respiratory tract regardless of their isolation source, indicating avirulence of APMV types 1–9 in mallard ducks. The link between the presence of a furin cleavage site in the F protein, syncytium formation, systemic spread, and virulence that has been well-established with APMV-1 pathotypes was not evident with the other APMV serotypes. The family Paramyxoviridae consists of enveloped viruses with a nonsegmented, single-stranded, negative-sense RNA genome [1] . These viruses have been isolated from a great variety of mammalian and avian species around the world. Many members of the family cause important human and animal diseases, while the disease potential of many other members is not known. The family is divided into two subfamilies: Paramyxovirinae and Pneumovirinae. The subfamily Paramyxovirinae comprises five genera: Rubulavirus, Respirovirus, Morbillivirus, Henipavirus, and Avulavirus. Subfamily Pneumovirinae is divided into two genera: Pneumovirus and Metapneumovirus. All paramyxoviruses isolated from avian species are classified into the genus Avulavirus, except avian metapneumoviruses, which are classified in the genus Metapneumovirus. Avian paramyxoviruses (APMVs) have been divided into nine different serotypes (APMV 1 to 9) based on Hemagglutination Inhibition (HI) and Neuraminidase Inhibition (NI) assays [2] . APMV-1 comprises all strains of Newcastle disease virus (NDV) and has been well characterized because of its economic importance in poultry industry. As an initial step towards characterizing other APMV serotypes, complete genome sequences of one or more representative strains of APMV serotypes 2 to 9 have been determined [3] [4] [5] [6] [7] [8] [9] [10] .