Poultry can be affected by a wide variety of diseases and parasites, some of which can be quite serious. Here are a few common diseases and parasites that can affect poultry:

1. Newcastle disease: Newcastle disease is a viral disease that affects birds and can be deadly for poultry. Symptoms include respiratory problems, diarrhea, and nervous system disorders.
2. Avian influenza: Avian influenza, also known as bird flu, is a viral disease that can affect poultry and other birds. It can be transmitted through the air or through contact with infected birds or their feces.
3. Salmonella: Salmonella is a type of bacterial infection that can affect poultry and cause food poisoning in humans. It is transmitted through contaminated feed or water, or through contact with infected birds.
4. E. coli: Escherichia coli (E. coli) is a type of bacterial infection that can cause illness in humans and poultry. It is transmitted through contaminated feed or water, or through contact with infected birds or their feces.
5. Coccidiosis: Coccidiosis is a parasitic disease that affects the intestinal tract of poultry and can cause diarrhea and weight loss. It is transmitted through contaminated feed or water or through contact with infected birds.
6. Lice and mites: Lice and mites are common parasites that can affect poultry, causing irritation and feather loss. They are transmitted through contact with infected birds or their environment.