WordPress spam has been worsening lately for everyone: It's an easy in for hackers, especially on dormant sites or unmonitored sites. I've cleaned tens of thousands of spam comments out of clients' sites before, and those comments not only show an a doorway for more spammers, but also bloat your site's database and ultimately slow you down. Which Google doesn't like. It's a chain reaction.
You can, however, combat it and automate many of these actions.
Check your discussion settings
First off, check your settings under Discussions (under Settings). Make sure you're comfortable with all of the settings here, they're your first line of defense.
Personally, I like to get an email for new comments, but on high-traffic blogs, this isn't feasible. You may want to enable approval of comment authors with a previously approved comment.
Akismet: Your next line of defense
On the plugin front, have you installed Akismet? And enabled it? Depending on your site's purpose, it can be as cheap as free or a small monthly fee. Installed with every single WordPress installation, this alone should give you a clue that this is a great way to combat spam. Be sure to tag those that are spam as such so Akismet learns. If you have hundreds or thousands of spam comments, it's easier and faster to go into the database and clean these up. (Side note: this may require a developer so you don't mess up your database!)
I also have been selecting "Silently discard the worst and most pervasive spam so I never see it." Not only does it sound wonderful, it does actually help.
reCAPTCHA their attention, not spam
Are you using reCAPTCHA on your comments? It's annoying to some who comment a lot, but it may be the answer for you. And not all options are impossible to decipher. Sweet CAPTCHA is a cute option, especially for food blogs. SI CAPTCHA is more standard-looking and has a lot of customization options, plus it works on Multisite and BuddyPress.
Ultimately it's up to you to combat spam and keep your site clean. Check in as often as you can – whether it's weekly, monthly or quarterly. After all, there might be a real conversation waiting for you.