Egg yolk peritonitis occurs in young flocks, at the start of production, when the flock shows a very fast increase in lay percentage. Often these flocks come out of rearing on bodyweight target, sometimes even 50-100 grams above bodyweight target but when these birds are light stimulated, they start laying very fast. In this situation, part of the follicles “miss” the oviduct and land free in the abdomen, with egg yolk peritonitis as result. Nervous flocks can also show some egg yolk peritonitis as a form of internal trauma then. One other form of internal trauma causing egg yolk peritonitis can be seen in birds housed on a aviary system which suffered from leg weakness; they show too tough “belly landing”. To conclude, egg yolk peritonitis can be a complication of viral infections targeting the ovaries, like IB, ART and ND.