Coccidia are intestinal parasites. They are very difficult to destroy and can be found anywhere where there are birds. During rear, the birds will receive coccidiostats in feed to reduce the amount of Coccidia they come in contact with. Some contact is needed to build up immunity.
Outbreaks occur when there is a reduced feed intake (less coccidiostats taken in), presence of other diseases or stressors, or a very high infection pressure (brooding in dirty houses). Birds will show watery diarrhoea, weight loss and high mortality if left untreated. Partridges in particular are very sensitive.