Welcome! Beautiful chicks. Only way to prevent Mareks Disease in chicks is purchase vaccinated birds on day one. Some hatcheries sell vaccinations to owners like Meyer Hatchery if hatch own to vaccinate. If in flock those who survive are most likely carriers for life and any new additions to flock especially if unvaccinated may die each year from Mareks Disease. Vaccination is key to prevention.
Another common disease is coccidiosis in chickens I’m sure you have read about and best prevention is medicated feed with amp until 8 weeks of age so will have protection once exposed to world outside the brooder where cocci can live. Good Luck! Have fun!
and Welcome to Backyard Chickens. 
