Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola, AIDS, avian influenza and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between Human Herpesvirus Six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. There is current controversy over whether the borna virus, previously thought of as causing neurological diseases in horses, could be responsible for psychiatric illnesses in humans.
Resistance to and recovery from viral infections will depend on the interactions that occur between virus and host. The defenses mounted by the host may act directly on the virus or indirectly on virus replication by altering or killing the infected cell. The non-specific host defenses function early in the encounter with virus to prevent or limit infection while the specific host defenses function after infection in recovery immunity to subsequent challenges. Although the host defense mechanisms involved in a particular viral infection will vary depending on the virus, dose and portal of entry, some general principals of virus-host interactions are summarized below.
BARRIERS TO INFECTION
The host has a number of barriers to infection that are inherent to the organism. These represent the first line of defense which function to prevent or limit infection.
The skin acts a formidable barrier to most viruses and only after this barrier is breached will viruses be able to infect the host.
Lack of Membrane Receptors
Viruses gain entry into host cells by first binding to specific receptors on cells (Table 1; adapted from: Roitt, Immunology, 5th Ed).