I usually recommend using a good quality poultry vitamin supplement to support their immune system and a probiotic or fermented feed to support their digestive tract. Providing them with plenty of space and good nutrition is important. If it was Marek's, others are likely to have been exposed. Some may already be infected but others will be resistant. The virus will lie dormant in any infected bird(s) until an outbreak is triggered, usually by stress, and what stresses one bird may not stress another, so you do not always see a number of birds all symptomatic at once. It is not even possible to tell which birds are infected until they eventually show symptoms (or die) which can be weeks months or years down the line. Like the human cold sore virus, it is a Herpes Virus and it is not infectious during the dormant phase, but once a bird is symptomatic, it is actively shedding the virus and the main route of transmission is via inhalation of infected dander dust.
Of course it may be that your bird died of something else and unless you have other birds exhibit Marek's symptoms at some point in the future we will never know, but the above suggestions of vitamins and probiotics and keeping them as happy and stress free as possible are simple steps that can only benefit them even if it is not Marek's. There is no cure for the disease itself.... just good management.
