Tomatoes can get a number of diseases, all documented very well on the internet. Most are fungal and spread by spores splashing on the leaves during rainstorms. Fungal diseases are also helped by high humidity and general wet conditions. Look up "early blight" and "late blight" for pictures and more information. There is nothing that can be done to help the infected leaves. picking them and disposing of them well away from the garden can reduce the spores spreading to other leaves.
Mulching is a big help too because it tends to separate the moist soil from the air, anything that locks moisture in the ground makes the air around the plants drier, and the spores are in the soil. not in the mulch on top (unless you mulch with last years tomato stalks - don't do that).
After frost kill them, thoroughly remove all the tomato stalks and and mulch and put it in the trash or burn it away from the garden. Whenever handling infected leaves, treat them as if they harbor a deadly disease, use gloves or wash your hands thoroughly.
Awesome information, thanks! We did not mulch them this year, but we will try it also next spring.