The medical term is Omphalitis and it is most commonly caused by Escherichia coli bacteria combined with Staphylococcus aureus, Streptoccus faecalis and other bacteria. Once they have it, there's no getting over it and it spreads pretty fast (about 1 to 2 days).
Omphalitis is a condition characterized by infected yolk sacs, often accompanied by unhealed navels in young fowl. It is infectious but noncontagious and associated with excessive humidity and marked contamination of the hatching eggs or incubator.
The affected chicks or poults usually appear normal until a few hours before death. Depression, drooping of the head, and huddling near the heat source usually are the only signs. The navel may be inflamed and fail to close, producing a wet spot on the abdomen; a scab may be present. Opportunistic bacteria (coliforms, staphylococci, Pseudomonas spp , and Proteus spp ) are often involved, and mixed infections are common. Proteolytic bacteria are prevalent in outbreaks. The yolk sac is not absorbed and often is highly congested or may contain solidified pieces of yolk material; peritonitis may be extensive. Edema of the sternal subcutis may be seen. Mortality often begins at hatching and continues to 10-14 days of age, with losses up to 15% in chickens and 50% in turkeys. Chilling or overheating during shipment may increase losses. Persistent, unabsorbed, infected yolks often produce chicks or poults with reduced weight gain.
There is no specific treatment; antibiotic use is based on the prevalent bacterial type involved, but is probably of little value. The disease is prevented by careful control of temperature, humidity, and sanitation in the incubator.