Tobacco mosaic virus is extremely persistent. You're right about not smoking while working in the garden, not dropping any cigarette butts in the garden, and washing your hands before touching the plants. When you do spot the disease in your garden, remove and destroy every bit of the infected plant. Wash your hands frequently and disinfect your tools by scrubbing them with scalding hot soapy water for at least five minutes. And perhaps the simplest advice of all: look for tomato varieties with a T on the label, indicating a resistance to the tobacco mosaic virus. Resistant varieties include Celebrity, Hawaiian, Sweet Chelsea, Park's Whopper, and Tropic.