This is a very mysterious 'alien'.  It looks very similar to oil and is very viscous.  (This means slimy if you don't know your science!)  It 'takes over people' by getting into peoples' eyes and nose.  The above picture isn't the best in the world but you can see 'black stuff' in his eyes.  That's how you can tell people have been 'got at'.  The government seems to know very little about it.  The Russians know a little but not really enough.  A cure is trying to be developed by both countries with-out success.  (File 11 throws more questions on to the already giant list I have!)

The oil has got into Krycek by accident and into Mulder deliberately.  There are tests being done on human subjects by putting the oil into them and basically seeing what happens.  I think this is the most mysterious and un-known alien in the X-Files.  I don't even know for certain if it is an alien, but it seems likely.

I'm sure in the future, the black cancer will crop up again raising more questions, (and answers to questions we hadn't even thought of!)