It may be that viruses are simply less of a concern: perhaps at that depth viruses have difficulty surviving outside of a host and so can't spread very easily?

It may be that viruses are simply less of a concern: perhaps at that depth viruses have difficulty surviving outside of a host and so can't spread very easily? It could even be that they lost their immune systems first due to a lack of genetic pressure to keep them, and this allowed their "fusing" behaviour to evolve.