Question: how do viruses become dangerous on animals?

Janet Daly answered on 9 Mar 2013:

Viruses cannot replicate by themselves so they have to hijack the replication machinery of their host’s cells…this can cause some damage to the host. Often, the damage caused will help the virus to spread to its next victim – for example the common cold virus makes us cough and this way it can be spread (through droplets in the air) to a new host. At the moment there is lots of norovirus around – this virus makes people be violently sick (which is why it gets called ‘winter vomiting disease’) and this is how it spreads to the next person. When viruses really become dangerous is when they find themselves in a new host that they are not adapted to. It usually is not in the best interests of the virus to kill its host because then the host isn’t able to replicate it any more, but sometimes they accidentally do this when they are not accustomed to the host. One example of this is Hendra virus in Australia – this virus usually replicates in fruit bats but does not make them ill. However, it sometimes gets passed on to horses (no-one is exactly sure how) and 3 out of 4 horses die as a result of infection. The virus can then be passed on to people in contact with the sick horses (like vets) and it can be deadly to them too.