Merek's disease is highly contagious and often fatal. It affects the nerves and organs and is caused by a herpes virus. Symptoms include lameness in one or both legs, paralysis, and drooping of the wings. In young birds symptoms include loss of appetite, depression, and sometimes tumors can be felt under the skin. There is no cure for the disease.
If you are having concerns whether you should vaccine or not, contact your local cooperative extension and find out if it is problem in your area and whether or not they recommend you get the chicks the vaccine.
I got the vaccine for my small flock. It only added $0.15 to the cost of each chick, and one lady that ordered with us lost her whole little backyard flock to the disease. After listening to her heartbreak, I decided it was a good idea. Anyway, the whole order had to have it, or none at all.