Bacterial diseases don't form---they exist and are spread. Salmonella is present in the sparrow population in Canterbury and can be carried by just about every animal you can think of including wild lizard populations and commercial poultry, and the pidgeons in the square used to be loaded with psitticosis (parrot fever). Campylobacter is frequently carried by birds and that is one reason it was not cultured in labs for a long time. Birds have a higher body temperatuture than us and they had no success until they raised the incubation temperature. You should always assume that animals carry diseases, and we don't quarantine humans arriving or returning to NZ (unless they have obvious signs of a quarantinable disease). There were only 7 internationally quarantinable diseases and now that smallpox has been iradicated there are only 6.