Coronavirus (Covid-19) is one of the largest global threats that we as humans have faced in centuries. But how does it infect the body and cause the symptoms that it does?
Covid-19 is part of a large family called ‘Coronaviridae’, characterised by ‘crown-like’ projections on its surface. Coronaviridae also includes diseases such as SARS (responsible for the 2002-2004 outbreak) and MERS.
The ‘crown-like’ projections play important part in the virus’ replication and survival. Viruses like Covid-19 are unique in the fact that they are ‘non-living’, so they quite cleverly manipulate a host cell’s replication and protein production ‘machinery’ to produce copies of themselves, which can then go on to infect even more cells.
How does Covid-19 actually enter a host? Well this is where the ‘crown-like’ projections come into play. These projections are called glycoproteins and have a specific amino acid sequence that enables them to bind specifically to structures on the surface of a host cell called receptors. Upon binding, the virus injects its genetic material (RNA) into the host cell. Imagine this like a lock and key: when the key fits (binds) in the lock, the door can be opened.
Some viruses e.g. coronaviruses have special modifications on their RNA, which means that there is no need for it to be converted to DNA to enable replication. The virus then manipulates the host cell’s replicative machinery to enable the production of viral proteins, which then assemble into a newly replicated ‘virion’. This ‘virion’ can then break off from the host cell membrane, enabling it to replicate even more. Symptoms associated with most viruses like Covid-19 stem from the fact that the host cells often die when the virion breaks off from the host cell membrane. Upon dying, stress signals are created by the host, often triggering an immune response. This is what causes symptoms in most cases.
The symptom of a persistent cough associated with Covid-19 is often due to a mucus build-up, designed to make it harder for the virus to attach to host cells, thus reducing viral replication. A high fever is designed to kill the virus, which denatures under high temperatures. Feelings of tiredness and weakness associated with many illnesses are due to the body prioritising fighting the virus instead of carrying out less important functions. Also, the build-up of white blood cells to fight off the virus may cause the feeling of ‘achy bones’.
As we have seen by the extremely high death rates due to Covid-19, in severe cases, the immune system can go into overdrive. In these cases, white blood cells active chemicals which can cause leakage of fluid into lungs, leading to pneumonia, which may lead to death. The combination of this and cellular destruction decreases the transportation of oxygen into the bloodstream from the lungs, leading to multiple organ failure without professional treatment.
However, patients often do not die of Covid-19 directly, but as a result of opportunistic bacterial infections, which take advantage of the weakened immune system, and can therefore cause serious and sometimes fatal harm.