It was some kind of dust, mist, blood, poison or something like that. They have different sindromes. Looks to me like your quite unlucky with yours, your guy is going to rot from the inside out, producing miasma as he waks around. Be very carful, as the FB extract is what causes it, and if the guy that killed it has any on him, then anyone who touches it will get the same syndrome's on whatever body part touched it, it a ton of blistered rotting feet making miasma everywere. Iv had a few forts taken down by these deadly syndromes.
As for a solution, the simplist way is to just quarenteen your guy and starve him out so he does not spread it, forbid the body and all the stuff in his invantory when he dies, (incase it has any FB extract on it, wouldent was some poor haulers to lose their hands) and wall off the exit to the tomb to never be touched again. The second, if you want your guy to live a bit linger, is to, once again, quarenteen him, but this time fill the chamber up with some water (4 max) to wash him off. When all the FB extract is off him and his stuff (check with [v] to view units, go over him, hit [I} for his invantory and if he has something like "Forgotten beast extract splatter on left upper arm" or another body part) get him outta there and wall the place off so no dorfs step in the FB extract. Quick warning, he will still rot and die, even with the FB extract off him, producing miasma as he does.
Hope this helps!
Edit; Wow, how could i forget to wish a very !!fun!! time! Well, anyway, orry i forgot, and be surw to have as much !!fun!! as possible! I hope its !!fun!!!