This was 1 year ago, and I've been working freelance since.
I've got an interview coming up for a full time role and was wondering what to say if they brought up why I left the job.
Lie or tell the truth?
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Honesty is the best policy.
If you illness was physical I don't see how this could be taken against you, especially that you have worked since.
If this was of a mental issue (e.g. depression) then you should be prepared to explain the steps you undertook to resolve the issue. (As pointed out in a comment this applies to a physical issues too)
Also remember the fact you lied about your medical issue may surface after you get a job - if you require time for checkups, rehabilitation, the illness comes back etc - if you ask for flexibility around this and it becomes clear that it's an old problem that you lied about you may face serious consequences.
Having said that, the amount of detail and how open you are about your illness will depend on the culture you are part of.