The Vorlons in the Babylon 5 saga are very secretive. As far as we know, only two humans ever visited their empire and lived.
Vorlon territory was restricted to other alien species and (almost) every known expedition into Vorlon space failed to return to their respective governments. Much like the Shadow homeworld, Vorlon is protected by an array of highly advanced and automated defense systems designed to keep out intruders and primitive spacefaring races. These defense systems remain in place and active even after the Vorlons abandoned their home and left known space; evidence suggests that the systems will not allow outsiders entry to Vorlon until 1,000,000 human years have elapsed.
They wear encounter suits and very rarely leave the suits because they don't want others to see how Vorlons appear.
Despite their secretiveness about their empire, their history, and especially their physiology, somebody poisoned the Vorlon ambassador to Babylon-5. How would anybody know how to poison a Vorlon?
In the pilot episode, The Gathering, ...
a member of the Wind Swords, a militant section of the Minbari warrior caste, disguised himself as station Commander Jeffrey Sinclair, and attempted to assassinate Ambassador Kosh with poison.
A person could not pick a chemical at random and hope it works. If they tried sodium-cyanide, they might discover it just makes Vorlons hallucinate, but does not injure them. Or if somebody tried, potassium-chloride, they might discover that Vorlons use that the way we use table salt, sodium-chloride.
Later in the episode ...
Doctor Benjamin Kyle decided to risk treating Kosh, which meant opening the encounter suit. Determining that a cure would be impossible without knowing where the poison entered the Vorlon, Dr. Kyle convinced newly arrived Psi Corps telepath Lyta Alexander to scan the Vorlon. Despite the very strict Psi Corps rules in such cases, Lyta performed the scan.
So we have a separate, but related question of how could a doctor know how to treat a Vorlon for poison if he doesn't understand Vorlon physiology, which poison was used, or the effect of the poison on the body? While this second question is intriguing, it's not the focus of this post.
I prefer answers backed by canon sources; such as comments from the producers, novelizations, or screenplays.