|Several possible reasons, such as:|
Virus is new and has not yet been put on the AV data base.
Virus of a type that keeps changing its form to fool AV software.
Particular AV is not a very good one.
Having an antivirus is a must but nothing is 100%. It is a battle against organised criminals, so careful browsing and being aware of what to do when things start happening (eg bogus AV pop-ups) are still valid.
Please come back and let us know the outcome.