Marek`s disease virus (MDV) may retain infectivity for months to years in feather follicle debris in the litter.Newcastle disease virus (NDV) is generally thermolabile, but may persist in the environment for up to 1 week.Infectious bronchitis virus (IBV) can survive in the environment from 12 days (in spring) to 56 days (in winter).Infectious bursal disease virus (IBDV) is very stable in the environment, with infectivity unaffected by pH as low as 2, and it persistently survives in poultry houses even when thorough cleaning and disinfection procedures are followed.Chicken anaemia virus (CAV) is extremely resistant to physical and chemical treatments. Inactivation of CAV in infective chicken products requires a core temperature of 95oC for 35 min or 100oC for 10 min. Some strains of fowl adenovirus (FAV) are extremely resistant in the environment, whereas others are heat and pH sensitive.