If I install a virtual machine
(VirtualBox) to test, and it does get
infected with viruses, will it also
infect my host system? In other words,
can I use the virtual machine for
testing without being concerned about
a virus on the virtual machine
infecting my host?
There seems to be some misconceptions about NAT and bridge connections in VM environments. These do not allow your host to be infected. A VM operating system will have no access whatsoever to the host operating system and will be completely unaware it is operating as a Client Virtual Machine. Software running inside that operating system will be even less wise about it.
It is through direct relationships between the client and the host machine that may exist a chance of getting infected. This happens if you allow the client and the host to share folders. The largest chunk of VMware (to name one popular product) vulnerabilities of note ever found have been directly or indirectly tagged to this feature. A complete isolation is achieved by turning off shared folders. Any other vulnerability has been discovered on the Host side when vulnerabilities on the VM engine itself would allow a potential attacker to hook up through the host machine and gain access to any clients, or run code of their own.
Security issues may indeed be more involving if one is running a large VM structure such as those proposed through VMware Server topologies. But if running single-computer VMware Workstation solutions, there is no security issue under NAT or Bridge connections. You are safe as long as you don't use shared folders.
EDIT: To be clear, when I speak of NAT or Bridge connections I'm speaking only of the VM ability to share the host network connection with its clients. This does not give the client any access to the host and it remains entirely isolated, provided functionality like VM Shared Folders is turned off. Naturally, if instead the user decides to network Host and Client, then said user explicitly decided to connect both machines, and with it wave intrinsic VM security. This then becomes no different from any other private network environment and the same securities issues and concerns need to be addressed.