IBV is a coronavirus that only causes disease in chickens, although some other birds may be subclinically infected. Some serotypes are geographically restricted, but multiple serotypes commonly cocirculate in one geographic region. In recent years, a novel IBV genotype, the QX strain, has become increasingly common in Asia and Europe. IBV is shed by infected chickens in respiratory discharges and feces, and it can be spread by aerosol, ingestion of contaminated feed and water, and contact with contaminated equipment and clothing. Naturally infected chickens and those vaccinated with live IBV may shed virus intermittently for up to 20 wk after infection. The incubation period is generally 24–48 hr, with the peak in excretion of virus from the respiratory tract lasting 3–5 days after infection. The severity of disease and the body systems involved are influenced by the strain of the virus; the age, strain, immune status, and diet of the chickens; and cold stress. In addition, coinfection with Mycoplasma gallisepticum, Mycoplasma synoviae, Escherichia coli, and/or Avibacterium paragallinarum can exacerbate disease.