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.
Answer: Virus.
This riddle appears in the following downloadable PDF files:
Can you solve one of the most famous logic puzzle ever?
Memory Matching game for kids, adults and seniors.
Connect the matching tiles and pop them all.