What is the difference between viral and genetic hydrocephalus?

Not much: Congenital hydrocephalus is usually caused by a blockage of CSF (cerebrospinal fluid) flow out of the brain (where it is produced) leading to excess fluid inside the brain. Infectious causes (often bacterial and parasitic) can clog the pathway out, just like congenital hydrocephalus, as well as limit the ability of the brain to resorb CSF after it completes its cycle around the spinal cord.

Difficult question: Babies can be born with hydrocephalus. The exact cause may not be able to be determined. Many are due to exposure to viruses in utero. Some are due to developmental defects of the brain. Some genetic disorders carry a risk for hydrocephalus as a consequence of the disorder. Some genetic disorders increase the risk for virus infections that can cause hydrocephalus.