In this case there would be spotting of the leaves, wilting, and other symptoms. The yellowing of the leaves would probably begin at the base of the plant and progress upward with black spots developing on the leaves. If these symptoms are present, you may have fungal disease in your tomatoes. Removal of the infected plants is still recommended, but some of the other information above changes. If your plants have fungal disease, it is unwise to plant tomatoes, or related plants such as chile and eggplant, in the same site this year or for the next couple of years.
Fungi can be soil-borne and spread from the soil to the plant.
Composting plants infected with fungi can also be a way of spreading the disease. If your compost gets hot enough, or if you treat the diseased plants to high heat before composting, you may avoid problems, but be careful. If you wish to compost plants which may have fungal diseases, you may solarize the plants to kill the fungi. Do this by sealing the infected plants in a black garbage bag which is then placed in sunlight. Allow them to remain in the sun for several days.