When the leaves get light and the veins remain dark it's called chlorosis, and it happens in plants that get too much water (and poor drainage) and not enough nutrients (usually iron).

If the roots are not getting some air because the soil is compacted around it, the roots get waterlogged and cannot take up nutrients.

So, chances are it's not a disease, but unfavorable environmental conditions that are causing the problem. You could try some fertilizer -- of the "quick" variety -- such as the kind that gets dissolved in water and poured into the soil.

Are these tomatoes in containers or in the ground? I'm sure they are quite large now, but is it possible to improve the soil situation? If they are in containers you might be able to gently loosen the soil, perhaps adding some good potting mix, but it would be better if you could replace the soil with a good potting mix, which is a daunting task with a large plant.

Also, if they are in containers, make sure the container is large enough to support the plant. Most tomato plants need at least a 5 gallon container in order to maintain health and have enough room for the roots.

If they're in the ground, I would suspect the soil is waterlogged and compacted, and there is not much that can be done, except to minimize watering and add fertilizer.

For right now, try the fertilizer. Again, not a slow-acting fertilizer, but a quick-acting fertilizer. They need nutrients quickly. If possible, improve the soil if the soil appears to be in rough shape.

The leaves in back look good, so it's not a dire situation and you can improve things quickly. There looks to be a bit of sun or insect damage on the chlorotic leaves, but nothing dreadful.

Cheers.

Chlorosis is simply a word that means "yellowing." The possible causes are many.

But the distortion is something more complex. Add virus and weedkiller to the list of possibilities.