Colibacillosis & Mycoplasmosis in chickens Avian influenza and Velogenic Newcastle Disease are the two most deadly diseases for chickens. H5N1 and ND viruses can kill many chickens in just a few days. Both agents are very frightening. However, colibacillosis and mycoplasmosis are the biggest economic losses for farmers. Because both of these diseases have a very high incidence rate. Maybe.