A detailed description of the experimental protocol, including animal sources, virus sources and the infection protocol has been published [23] . Briefly, 34 Chinese-origin M. fascicularis were assigned to five experimental groups matched for age, weight, and gender to the extent possible. Four groups of eight animals were inoculated through the combined intratracheal, intranasal, tonsillar, and conjunctival routes with a total of 10 7 Two animals per group were euthanatized at days 1, 2, 4, and 7 postinfection (pi). Two animals were used as mock-infected control animals and terminated at day 7 post-challenge. At necropsy, all tissues were examined grossly and harvested. Samples for histology and immunohistochemistry were fixed in 10% neutral-buffered formaldehyde for 48 h, followed by transfer into 70% methanol-free ethanol, and stored until processing. The tissues were routine embedded in paraffin and sectioned (4-5 μm). Hematoxylene-and eosin (HE)-stained sections were examined on an Olympus BX41 microscope equipped with a Q-Color3 camera (Olympus) and corresponding computer software. Immunohistochemistry (IHC) detection of influenza virus antigen in formaldehyde-fixed, paraffin-embedded tissues was carried out as described previously [22, 23] , except that proteinase K was used for antigen retrieval. The H1N1 viruses were detected using the influenza virus nucleoprotein-specific monoclonal antibody M322211 (Fitzgerald Industries, Concord, MA, USA). Because this antibody appeared to cross-react poorly with the H5N1 nucleoprotein, a polyclonal rabbit antibody specific for H5N1 viruses (source: CDC) was used on tissues of the H5N1 group. Tissues from our previous studies were used as positive controls [22, 29] . Other controls included substitution of the influenza-virus-specific antibodies with monoclonal antibodies specific for various flaviviruses (West Nile virus, St. Louis encephalitis virus, bovine pestivirus) and omission of the primary or secondary antibodies.