A virtual machine does not access directly the memory of the host in case of shared folders. In reality, the virtual machine uses a special file system driver in the Guest Addition to talk to the host. A shared folder is implemented like pseudo-network redirector.
The only way to be sure a virtual machine won't infect its host is to develop an architecture that isolates the virtual machine from its hosts as if it separates them physically. As long as this has not been possible (AFAIK) it is not strange to read many cases on the Internet of hypervisors being escaped by malware (IBM X-Force ® 2010 Mid-Year Trend and Risk Report).
When sharing folders between the host and guest, you still have the option (at least with VirtualBox and VMWare Player that I use) to force the guest to access the shared folders in read only mode (you can copy data from the shared folder to your virtual machine but not in the opposite direction).
As for your question's title, no, shared folders do not add any security layer regarding the possibility of infection of your host. In opposite, it may be an additional attack vector especially in the case you create a shared folder with the default read AND write option.