Star Trek: 10 Shapeshifting Aliens That We Know About

'Holding onto anger is like drinking poison and expecting the other person to die,' as the saying goes, but you can hardly blame this offshoot of the Great Link for bearing a grudge. The Founders started it but Section 31, with the agreement of the Federation Council after the fact, did some (mass biological) poisoning of their own in a genocidal attempt to stop it.

And then there was 'Project Proteus,' Section 31's next wartime crime, named after the prophetic Greek god of the seas and OG shapeshifter. Begun as a series of experiments on captured Changelings at Daystrom Station in order to create a band of super spies, the result wasn't extra-effective espionage, but Vadic with a vengeance, reborn with some rather scary new abilities that she could then, upon escape from Daystrom, pass on to any Changeling wanting to join her breakaway cause (of death to the Federation, hand-in-assimilated-hand with what was left of the Borg).

Let's have a look at Doctor Crusher's autopsy report on the dead Changeling still in the form of Ensign Sydney La Forge on the Titan-A for a better idea of Vadic et al.'s Section 31, thelomium-847-given capabilities:

[The subject has] been able to completely replicate internal human organs. Only under intense dissection does the tissue revert to its natural state. 

Such perfect replication also included the person's blood plasma. Along with the post-mortem retention of form, that meant these Changelings could pass the standard blood screening tests, although those never seemed to be much of a hindrance for the Founders in the first place. No need for concern, however. Admiral Crusher found a workaround!