Riddle

I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough. Faster than a twitch!

Choose an answer

A.Bacteria
B.A germ
C.Cancer
D.A virus
Hint: It's a 2-word phrase with 6 letters, starting with "A".

Answer

A virus

Why this is the answer

A virus is microscopic yet can be deadly ("mark your end"), and it works against humans rather than for them, like a tiny, tough foe.

This riddle is filed under Hard/Difficult Riddles and rated Medium.