viral pathogenicity in hosts. Virus factors encoded in the genome often control the tropism, routes of virus entry, shedding and transmission. In polioviruses, the attenuating point mutations are thought to induce a replication and translation defect to reduce the virus' ability of cross-linking to host cells and replicate within the nervous system. Viruses have also developed a variety of immunomodulation mechanisms to subvert the host immune response. This tend to feature virus-encoded decoy receptors that target cytokines and chemokines produced as part of the host immune response, or homologues of host cytokines. As such, viruses capable of manipulating the host cell response to infection as an immune evasion strategy exhibit greater pathogenicity. Host factors Viral pathogenesis is also largely dependent on host factors. Several viral infections have displayed a variety of effects, ranging from asymptomatic to symptomatic or even critical infection, solely based on differing host factors alone. In particular, genetic factors, age and immunocompetence play an important role is dictating whether the viral infection can be modulated by the host. Mice that possess functional Mx genes encode an Mx1 protein which can selectively inhibit influenza replication. Therefore, mice carrying a non-functional Mx allele fail to synthesise the Mx protein and are more susceptible to influenza infection. Alternatively, immunocompromised individuals due to existing illnesses may have a defective immune system which makes them more vulnerable to damage by the virus. Furthermore, a number of viruses display variable pathogenicity depending on the age of the host. Mumps, polio, and Epstein-Barr virus cause more severe disease in adults, while others like rotavirus cause more severe infection in infants. It is therefore hypothesized that the host immune system and defense mechanisms might differ with age. Disease mechanisms: How do viral infections cause disease? A viral infection does not always cause disease. A viral infection simply involves viral replication in the host, but disease is the damage caused by viral multiplication. An individual who has a viral infection but does not display disease symptoms is known as a carrier. Damage caused by the virus Once inside host cells, viruses can destroy cells through a variety of mechanisms. Viruses often induce direct cytopathic effects to disrupt cellular functions. This could be through releasing enzymes to degrade host metabolic precursors, or releasing proteins that inhibit the synthesis of important host factors, proteins, DNA and/or RNA. Namely, viral proteins of herpes simplex virus can degrade host DNA