I'm talking specifically about blight. If your tomato plants had blight last year, chances are good that they'll get it again this year.
Tomato blight is caused by several kinds of fungi.
It usually starts in the soil and spreads quickly up the leaves. Spots appear on the leaves, and eventually, they shrivel up and fall off. Your poor tomatoes will be left hanging all alone with no protection. The best way to prevent blight is to simply plant your tomatoes in a new location. But sometimes that's not an option.