Virtual Machine and Virus

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.

It depends.

If your virtual machine (guest) has no network access to your host, your host won't get affect by any virus in your guest operating system.

My 2 cents...

In a nutshell, malware that executes in the context of the guest OS will NOT be able to infect the host OS, and will likely not even be aware that there is a host OS (though, hypothetically, breaking out of the virtualized environment IS possible, it won't become very common for a while, I suspect).

Some exceptions:

Overall, web surfing in the context of a VM is probably the safest way to surf, hands down (given the poor track record of AV s/w and other avenues of protection). In fact, using a separate, restricted account is probably sufficient, but a VM will certainly provide additional isolation.

No, if you don't setup any network connection (like NAT or Bridge) between host and guest OS. If you want to ensure total separation between the two worlds, please prefer "Bridge" connections and map one NIC to your Host PC and one other NIC to your VM-ed Guest.

It would be like having two isolated networks sharing only the powering bus (your actual PC, indeed).

VirtualBox, but also VMWare or Xen or Parallels, can easily setup for you such an environment