Does your basil look like this? Then I'm sorry to say your plants are likely to have downy mildew, a fungal disease that's fatal to our favorite pesto ingredient. I found it on some of the Italian basil plants in my community garden plot yesterday, and had to pull them out. (Luckily I was able to bring them home, and throw the leaves into the food processor with pine nuts, garlic, olive oil and a touch of lemon juice. Humans - and pasta - are not affected by downy mildew.)
I'm hoping that prompt removal will mean the disease doesn't spread to my other basil plants, which are in a sunnier and airier location. Plant crowding is a big factor in susceptibility to downy mildew (along with wet weather), and my affected plants were a bit close to my peppers and my neighbor's encroaching tomatoes. We had downy mildew on basil in the demo garden a couple of years ago, and I noted the same crowding issue then. Also, it seemed that other types of basil - lemon, Thai, purple types - were less susceptible than Italian. Of course that's the kind I want most of!