Viruses, Harmful Bacteria and other illnesses plague every species on earth and chickens have their fair share of them too.
====== Viruses ======
EAV-HP retrovirus:ever seen your chicken lay blue eggs? In some breeds this is normal but in most breeds this is caused by the EAV-HP retrovirus. It inserts a gene that turns eggs blue. This gene is called called oocyan. It's weird to see blue eggs but it isn't harmful.