Mycotic abscesses are due to
Bacterial infection
Fungal infection
Viral infection
Mixed infection
As the name suggests, mycotic abscesses are caused by fungal infections.
However remember that mycotic aneurysm is a misnomer and is caused due to bacterial infection of arterial wall. The infection weakens the wall to cause aneurysm formation (Organisms are usually Staphylococcal, streptococcal, or salmonella)