Coronaviruses enter the human body by being inhaled or via direct touch to the mouth, nose and eyes. They bind to and infect the cells lining the upper and lower airways and lungs. On average, symptoms will develop five days after infection, but this can range from two to 12 days. The time between infection and symptoms developing is called the incubation period. In an unknown number of people, the infection may resolve itself without the individual experiencing any symptoms at all. This is probably due to a fast and effective response by the immune system.
Symptoms generally occur after the virus causes direct damage to the cells of the airways and lung, or when the virus triggers an immune response. Irritation of the airway produces a sore throat and cough and sometimes a blocked or runny nose. A cough is a reflex to clear the airway of perceived phlegm, though COVID-19 usually produces a dry cough.
As part of the immune response to infection, signalling molecules called cytokines are produced. Cytokines help to mediate immunity through communication between cells, but they can also have a number of adverse effects during the course of illness. They contribute to fever and fatigue, muscle aches, headache and a loss of appetite. Diarrhoea, nausea and vomiting are rarer but may occur when the virus is present in the gut. The duration of symptoms ranges from one to three weeks depending on the severity of illness.
About 80% of people with COVID-19 have mild disease and never require hospitalisation. For those who do develop more serious illness, it is most frequently a form of pneumonia. This is where the body’s immune system fighting the virus causes inflammation and damage in an area of the lung. These areas are visible on X-ray or computerised tomography (CT) scans as white patches.
In COVID-19 pneumonia usually appears in patches in the lower parts or outer edges of both lungs. Even in people without symptoms, pneumonia sometimes shows up on CT scans. In most cases of COVID-19, the pneumonia resolves completely, often without serious illness. However, sometimes the pneumonia can worsen, causing severe disease.
One of the defining features of severe disease is shortness of breath. When pneumonia extends to a large enough area in the lungs, it prevents the lungs from functioning normally so less oxygen can enter the blood. The body responds by breathing faster and this is experienced as feeling short of breath. At lower oxygen levels, the affected person can look and feel drowsy. Nurses and doctors look out for severe disease by measuring breathing rate and oxygen levels in the blood. Severe disease should be identified early because patients may need hospital treatment, rather than having to self-isolate at home. Among severe cases, approximately one in ten will not survive.
Acute respiratory distress syndrome (ARDS)
In a proportion of patients with severe disease, the virus over stimulates the immune system, rapidly causing damage to the gas exchange sacs in the lung. These sacs start to leak and fill up with fluid, which prevents oxygen from passing into the blood or waste gases from passing out. ARDS is usually detected because the individual becomes severely unwell over just a few hours. Doctors can use a chest X-ray or scan to identify ARDS because they often show changes that are distinct from those caused by pneumonia.
Injury to the heart
In critical illness with COVID-19, the heart can be affected too. In the worst cases of this infection, the heart may be damaged but it is not yet known whether this is due to direct effects of the virus or to the body’s response to the presence of virus in the lungs.