Hi I was just wondering say a new virus came out but was only made to infect a few computers how would avast or any other antivirus detect it.
Insufficient date to compute ;D
Too many permutations to speculate, method of delivery, method of infection, how it would limit to a FEW computers, etc.
All new viruses have to be detected or suspected (extra process running, etc.) by the user or their system security policy and then reported. Heuristics, generic detection, suspicious traffic, etc. would be the most common detection methods.
So it could take some time until then the human brain
A very prescient question! In fact virus writers are now limiting their viral creations to the number of computers they infect. A global epidemic means anti-virus programs add the virus to their definitions but a limited outbreak means the virus remains undetected by most anti-virus programs.
Why is this important? Because virus writers now work for criminal organistions and try to create zombie computers to do their masters’ evil bidding. 5000 computers is quite enough to carry out a DDOS attack for example in punishment for non-payment of a blackmail demand.
Those who work for anti-virus programs now have to go out and seek new malware where it is spawned in the stygian depths of the internet:
http://news.com.com/Experts+Zombies+ousting+viruses/2100-7355_3-5720428.html?tag=st.rn
Well avast! only use heuristic for emails, avast! Standard Shield doesn’t support heuristic so it won’t scan any unknown virus that is not in it’s Virus database. But avast! staff are working really really really hard finding new viruses and giving us updates. 
So then how do the people who work for the virus company get samples of the virus?
Some do it by surfing the web unprotected 24/7.
How does avast get its virus info.
I would imagine the answer is roughly:
By ways many and secret.
And none of them yours, mine or any passing virus writer’s right to know.