Yes even wordpress server installations can get viruses! I learned the hard way after recieving a virus.  This virus gave everyone that visited some of my sites either a virus or their anti virus picked it up.

How You Get a Wordpress Virus?
A wordpress virus can be installed simply by logging into your ftp account after already receiving a virus on your computer. It will then try to install itself on your wordpress installation files.  Usually you’ll find a strng of code in page.php which will load itself each time someone opens up your website.

Risks of a Wordpress Virus?
Sure there are plenty of different risks that come with getting a wordpress virus. Google could block your site in their index for being an attack site.  You’ll lose visitors and loyal users because they’ve been infected from you.  Longer load times on your site.  The fact that someone else has outside access to your site.

What To Do In Case of a Wordpress Virus
First thing is first, change all your passwords, ftp, user login, everything. Try installing a fresh Wordpress Installation, this might remove the virus.  If all things fail, seek professional help.