Diesel bug is the common name for microbial contamination in diesel type fuels. This contamination is caused by microbial bacteria that enter the fuel tank and feed on the fuel, it has become much more common as modern fuels now contain larger percentages of bio diesel components.
Water caused by condensation will also attract the growth of micro-organisms within the water phase, these same organisms will use the fuel as a nutrient source creating gums and resins building up as a layer inside the tank and fuel lines.