Viruses can't turn into bacteria because they don't have the molecular machinery to effect any change in their own structure - they're just DNA or RNA sheathed in a bit of protein with none of the requirements for self-replication. No ribosomes, no nothing. They rely on taking over other cells and hijacking their machinery to live.

Bacteria can't turn into fungi because they're as different from one another as plants are from animals. More so, actually, in a genetic sense. Many bacteria are even prokaryotic, meaning they don't have cell organelles that fungi need for reproduction. Even exposed to powerful mutagens it would take hundreds of millions of years for that large an evolutionary leap.

Imagine how unlikely it would be to spill some oil on yourself and suddenly turn into an oak tree. That's slightly more likely than bacteria turning into fungi.