Unless you are not using the host OS for anything _other_ than virtualization. If the host OS is used to host VMs[1], which are then used for specific tasks (casual browsing, banking, development, etc). Any exploit will be limited to the VM. This would be a pretty solid setup. It is only vulnerable to attackers that have direct access to the hardware, or have the ability to exploit the hypervisor.

[1] in other words if the host OS is used as a hypervisor, or if the host OS _is_ a hypervisor.