Disclosure may not be a good idea if you think your employer might treat the information insensitively or if you think personal information could be made public, or that you may suffer discrimination. On the other hand if your employer is supportive it could be easier to make changes to your workload, to have time off for appointments or to deal with periods of sickness.
When applying for jobs, there may be questions on the application form about whether you have a medical condition. If you lie and they later find out you have HIV, this is called a breach of mutual trust and you could lose the job. As an alternative you could try as tactfully as possible to avoid the question: