There are many diseases that can affect chickens, some of which are highly contagious and can spread rapidly among a flock. Some common chicken diseases include:

1. Avian Influenza (Bird Flu)
2. Newcastle Disease
3. Fowl Pox
4. Marek’s Disease
5. Salmonellosis
6. E. coli Infection
7. Enteritis
8. Chronic Respiratory Disease (CRD)
9. Mycoplasmosis
10. Lice and Mite Infestations

It’s important to take proper precautions to prevent the spread of chicken diseases, such as biosecurity measures, regular cleaning and disinfection, proper nutrition, and regular veterinary check-ups. If you suspect that your chickens may be sick, it’s best to consult with a veterinarian for proper diagnosis and treatment.