In The X-Files S05E11, Two Fathers, Agent Jeffrey Spender is asked by the Cigarette Smoking Man to prove his allegiance once more, and ordered to kill a rebel alien who is masquerading as a member of the Syndicate.
|Spencer being tasked by his father with killing the rebel||Spencer confronting the disguised rebel alien|
When Spender's attempt fails, and Alex Krycek appears and kills the rebel, the green fluid doesn't affect Spender in the slightest (nor does it affect Krycek, but there are plausible explanations for that, including his infection with the Black Oil).
Normally, the alien green blood, which contains a retrovirus, infects people when exposed to it, causing them to suffer corrosive damage, which is lethal unless treated (a cold environment (of <2° F, IIRC) delays the blood's effects).
Why was Jeffrey Spender not affected by the green blood?