"Diesel bug" is actually due to several groups of micro-organisms, including bacteria, yeasts and fungi that can grow in diesel because it is an organic fuel. Unfortunately your diesel tank can be an ideal place for these organisms to grow as it is an aerobic environment where they have water and a food source (carbon + trace elements) for germination and growth.

That's bad enough, but their size means these micro-organisms can live in small crevices in your tank, in water droplets and can be found even in the air, so avoiding contamination is hard, and getting rid of it even harder. Apart from keeping your tanks topped up and removing any accumulated water, there is little you can do to prevent it without diesel polishing. Chemical treatments help, especially if the bug numbers are kept in check, although once the cells are dead, they unfortunately clump together and produce a slimy mess in your tank that needs to be removed or it will block filter after filter and act as a surface for further growth.