Given the way that news gets around, it is unlikely that you will be able to tell one colleague without others becoming aware of your situation quite quickly. Despite your wishes, sometimes it can even happen that information from outside your work situation alerts colleagues about your MS unintentionally, for example an inadvertent message from a family member to a colleague about an absence from work. So it is probably wise to work out ways in which to tell your colleagues in a planned process.
Although most of your colleagues will have probably heard something about Multiple Sclerosis, their views will be based on a wide range of experiences and ideas, and thus may not be accurate. This will not help in the understanding of your condition. So you will need to do more than simply indicating your diagnosis. The best thing may be to give each of your colleagues some written information about Multiple Sclerosis – perhaps one or more of the pamphlets on MS available from the MS Society – at the time you are informing them about your situation. They can then have accurate information, and you can respond to any questions that they might want to ask you about your own MS. It may be worth reminding them, if they were not aware of your Multiple Sclerosis before you told them, that this shows how little your work, and your working relationships with them were affected – and indeed this may continue for a long time.