If food is left out overnight, or out for a time to go rancid, bacterial/fungal infections can take place in the intestinal tract. Wild birds can transmit worms, coccidiosis, and Cholera. Rodents can bring Cholera and worms into the flock as well. Some strains of Cholera will make an otherwise healthy bird die the next day. All will show diarrhea as a symptom.
When an intestinal disease has enough time to damage the intestines (enteritis), it is often too late. Moulting birds are more vulnerable to sickness if their immune system is compromised. Harsh weather is stressful to birds, as is the increased population of parasites and protozoa in damp environments. Much energy is put forth to form new plumage during moult and nutritional requirements are greater. Supplementing vitamin-mineral-probiotic dispersible powders in waterers is a common practice with my flock. I do that 3 days a week.
There are many wormers on the market. Some are broad spectrum and others target only a few types. using the same wormer year after year is also a problem since nature's parasites can build a resistance to them. Albendazole, Fenbendazole, and Praziquantel-Pyrantel are very affective and safe wormers to use when used properly. Valbazen liquid suspension(Albendazole) and Safeguard liquid (Fenbendazole) labeled for goats both require a dose of 1/4 cc for Bantams/ 1/2 cc for Standard adults given orally by a syringe without a needle. Then another dose is given 10 days later. There is a 14 day withdrawal for eggs. If you haven't wormed your birds, I would recommend doing so. If one chicken is carrying them, the others likely do too.
Coccidiosis also thrives in moist environments. I use Corid as a preventative in waterers when raising chicks for at least the first 7 months of life since I don't use medicated feed. When birds lose weight and become anemic, often mites, lice, intestinal worms or Coccidiosis protozoa are the suspects. This is why checking for external parasites like mites or lice, then worming, then treating for Coccidiosis are often followed. Let me know what you observe after looking your birds over, and I'll help the best I can.
