Simple; when used as intended then the only thing which is directly at risk is your own data.
Normally you'd log onto FreeBSD as a regular user and such users can, by default, not access any system binaries for writing. As such there's no direct risk of getting infected with a virus.
But that doesn't mean that there aren't any risks at all. For example, your local data (your home directory) also contains login scripts. Therefor it would be theoretically possible for a malicious program to alter that. For example by placing an executable
su file somewhere in your home directory, then changing the search path so that this file would be discovered first. The program in question is obviously meant to capture your root password.
Of course it remains to be seen what an outside attacker could manage with having that password, but that's another story.
Another aspect is the connection you have with the Internet. Normally you'd be behind a router which is usually configured to allow data to go out (from your computer to the Internet) but not back in. Only data which you requested yourself. As such it's also not easily possible for an attacker on the Internet to directly access your machine.
This is of course heavily dependent on the way your Internet connection is configured.
The most important aspect of them all though: security is
not something you simply install or apply. Security is an ongoing process to ensure that your system remains safe.
Chapter 13 of the FreeBSD handbook is a good read for this.
