MEGAN Assignation after MALT

Hello, recently I used MALT to make a viral database with whole genomes, when I run MALT with my samples, I get the expected assignations of taxa, nevertheless there a couple of reads corresponding to bacteria or even eucarya, How is this possible if my database only contains viruses?. Thanks.

If you provided mapping file to MALT to identify taxa, then this could caused by discrepancies between the mapping file and the database.
If you requested that MALT use name parsing to deter mine taxon identity then this could be due to the same taxonomic name occurring multiple times different places in the taxonomy.