It’s very common for viruses to get bad press, take the recent story from Australia where doctors urged the public to get flu vaccinations after 68 people were confirmed to have been killed as a result of an “unprecedented” flu season – a seasonal outbreak of the Influenza A virus. How do viruses cause disease and why are some viral diseases worse than others?

Virulence is a term that describes the relative ability of a pathogen (or infectious agent) to cause disease. Some viruses can be highly virulent, which means they cause severely debilitating disease, some can be avirulent and replicate without causing disease at all, while others sit somewhere in the middle. Viral pathogenesis describes the process by which disease is caused by an infection of a virus or virions (which are infectious viral particles).

The virulence of a virus is dependent on its genetic make-up; some viruses had genes that encode proteins which alter its ability to replicate (often making the virus less virulent), and some viruses had genes which encode for proteins which alter the host’s mechanism of defence and immune response. Some viruses contain genes which enable their spread within the host, and some have genes which encode toxins which cause host cell damage or death.

Viruses need a host cell to replicate; to do this, viruses use receptors on their surface to bind to their target cells. In humans, these cells are usually those of the skin, respiratory, gastrointestinal, and genital tissues. The binding to their target cells allows viruses to enter the cells. Once a virus is inside its host cell, the virus hijacks the cell’s replicating machinery, making copies of the virus. These copies then infect neighbouring cells, and the process begins again; the copies of the virus can also escape to the environment and infect more people.

The severity and symptoms of the viral infection is dependent on the types of cells that it infects – this is called tissue tropism. Influenza A, the virus which causes the seasonal flu, targets cells in the respiratory tract and can therefore damage the respiratory tract and lungs, sometimes causing respiratory failure. HIV, the virus which causes AIDS, targets immune cells with a specific marker (a glycoprotein known as CD4). HIV binds to those immune cells, and either waits before infecting, or infects and kills those cells, leading to failure of your immune system.

Viruses can cause cell death in a number of ways. Viruses can divert the cell’s energy, shut off its synthesis of macromolecules, decrease translation of the cell’s mRNA by ribosomes, and outcompete the cell for viral promoters and transcriptional enhancers. Viruses can also induce mutations in the cell’s genome, cause inflammation, and initiate the immune response.

Viruses persist in our body and continue to multiply if the immune response is ineffective in eliminating them. Viruses can be eliminated using cytotoxic T cells and natural killer cells, which kill cells infected with viruses using toxins. Antibodies eliminate viruses before they even infect cells; antibodies detect the presence of a virus in the body, bind to the virus, prevent its binding to host cells, and initiate a process called phagocytosis. This is where a cell, a phagocyte, engulfs the virus and destroys it.

If the immune response is insufficient, the virus persists and continues to use host cells to replicate, often killing the cells in the process, leading to progression of disease and spread of the virus. Different types of viruses affect humans in different ways, while some viruses cause severe damage to the body, some infections often go almost unnoticed.