Apparently he has been (or at least acting to be) sick for quite a while. Notice how we never hear what is wrong with him - no diagnosis, that I’ve seen.
Maybe he’s not sick and just dodging his legal troubles.
No diagnosis, because no doctor called in to make the diagnosis. No diagnosis, no treatment. The Brits don’t want Assange to testify, any more than the Boston branch of the FBI want Whitey Bulgar to testify in 2019 (he was going to).
While at the Ecuador embassy for 7 years, his illness developed. Again, no doctor, no diagnosis, no treatment. Could be untreated diabetes or untreated high blood pressure. Poison, with no doctor, thus no diagnosis and no treatment, cannot be ruled out by an outside observer, but I consider something simpler like diabetes to be more likely.