Antivirus question I dont knwo the answer to

modern anti-virus programs use cryptographic hash
functions to detect virus infection. Namely, an anti-virus program first applies a hash
function to compute a checksum of each file, and store the checksums in a secret place.
When scanning for viruses, for each file the anti-virus program computes its checksum
again, and compares this checksum with that of the file that has been computed and
stored.
a. What property of the hash function is needed in order for this approach to work?
b. How can a virus program avoid such detection?

 

Well I am in an information security class and we are supposed to find a way to answer this question and I do not have a clue.