Emerging and reemerging diseases especially among infectious diseases in humans and animals including birds are a natural phenomenon. These diseases emerge for a various reasons such as mutations (genetic drift), recombination (genetic shift) and reassortment of genes which may be influenced by selection pressure, environment, management, etc. Some of the pathogens can be regional, occurring in a certain geographic location of the world or within the same country. It is also probable that certain pathogen may be present in a bird population but could not be identified because of lack of technology. A good example of this is discovery of avian Bornavirus, the causal agent of proventricular dilatation disease, which was only discovered in 2008 by theuse of metagenomics even though the disease has been known since 1990's or earlier. This review will deal with certain diseases such as coxiellosis, other bacterial diseases due to Yersinia pseudotuberculosis, Klebsiella pneumoniae, Aeromonas hydrophila, an unidentified silver positive filamentous bacteria and others, as well as, protozoal diseases caused by Sarcocystis calchasi and Trichomonas gallinae and viral diseases caused by adenoviruses and a novel psittacine herpesvirus.
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