What is a Virus? What are Viruses made up of ?

5. How are viruses transmitted?

There are many different ways in which a virus can spread from person to person. The mode of transmission is usually related to the location of virus replication. A virus will only infect cells that possess the correct surface receptors. For example gastro-intestinal viruses target and replicate in the cells that line the gut, causing damage which leads to diarrhoea. The newly-replicated virus progeny will be shed in the diarrhoea, and can be spread via the faecal-oral route due to poor hygiene or through contaminated water systems. Viruses such as HIV infect and replicate in specific immune cells and are present in blood (CD4+ Tcells or macrophages). They can only be spread by direct contact with blood or other bodily fluids e.g. during un-protected sex. Respiratory viruses, such as SARS-CoV-2 or influenza virus infect and replicate in cells that line the airways. Virus progeny are shed into respiratory droplets which are aerosolised and spread to others via a cough or sneeze. Virus-loaded droplets can travel ~2 metres and remain for some time on surfaces that they come in to contact with. This means that SARS-CoV-2 can transmit from one person to another directly by inhalation of virus-loaded respiratory droplets but can also be transmitted by hands that have touched contaminated surfaces. This is why it is so important to contain coughs / sneezes, dispose of tissues properly, and to maintain strict handwashing policies.