In some diseases you may be able to institute a cure if you can catch it early enough ie relatively small numbers, isolated or confined population etc. Also TRY to prevent incoming birds from contact and prevent sick birds from scattering. Losses in 10's of thousands. Clostridium botulinum - anaerobic bacteria toxin.
Bacteria infest the rotting vegetation, crustaceans, fly maggots and later duck carcasses.
Can't walk but pull themselves with wings. Oral administration of fresh water. Shelter Class 3 birds- completely paralysed. Remove and burn carcasses. If only dabblers are involved and it is also impossible to drain, deep flooding may be a help.