Anatomy of an infection
How does the coronavirus infect the body, and what happens when it does?
What exactly is a coronavirus?
The Nobel biology laureate Peter Medawar called a virus “a piece of bad news wrapped in protein”. Viruses are coiled strands of genetic material, in a protective coat of protein (and sometimes an envelope of fat), which invade living cells in order to replicate themselves. Coronaviruses are “unimaginably small”, said Tom Whipple in The Times. “If you packed a couple of trillion together they would fill a pinhead.” Viruses are “not exactly live but not exactly dead”, existing “between the atomic world and the world of the animal”. They’re not organisms: they can’t reproduce or produce their own energy without a host cell. Coronaviruses are so called because they are covered with knobbly spikes, giving the appearance of a crown or corona. Sars-CoV-2 is thought to have originated in bats and to have jumped to humans via an intermediary, perhaps a pangolin.
How does the virus infect humans?
Those infected with coronavirus soon carry trillions of microbes; their saliva teems with them. When they cough, sneeze, talk or even just breathe heavily, they emit aerosols: droplets laden with viruses. A study of a Covid-19 ward at the University of Nebraska Medical Centre found that the virus was everywhere, even in a well-cleaned facility: on iPads, spectacles, surfaces, bedside tables, toilets. The virus had invisibly occupied the whole floor, and could be found in two-thirds of air samples taken in the hall outside. Many things can kill the virus; soap destroys the delicate fatty envelope surrounding the coronavirus. Even so, Covid-19 viruses have been found to survive in the air for several hours. On plastic and stainless steel surfaces, viable viruses have been found after three days. The viral particles enter the human body via the eyes, mouth or nose, then latch onto cells in the respiratory tract. The virus’s spikes fit inside a receptor on certain cells, and open them up, like a lock in a key. Once inside it takes control.
How does it take control of cells?
The virus has only one aim: to copy itself. So once it attaches itself to a healthy cell, it sets about its work, fusing its membrane with the cell membrane, and releasing its core genetic material, its RNA. The cell assumes that the RNA is a message from the DNA at its own core, and starts cranking out copies of the coronavirus. In effect, said William Schaffner of Vanderbilt University, the intruders order the inflamed cell, “Don’t do your usual job. Your job is now to help me multiply.” Cells do this and usually die in the process.
What does this do to the body?
Average incubation period, the time from infection to showing symptoms, – typically fever, a dry cough and fatigue – is five days; though it can take as little as two days or as long as two weeks. About 80% of people who develop Covid-19 recover without needing treatment. There are also an unknown proportion of “subclinical” or asymptomatic cases: screening programmes in Iceland and parts of Italy have suggested that at least 50% of those infected may show no symptoms. This may partly explain the virus’s swift spread around the world. In mild cases, the infection stays largely in the upper respiratory tract; as the immune system makes antibodies and activates T-cells that neutralise and clear the virus, the victim recovers in a couple of weeks. In more severe cases, the invader pushes on to the lower respiratory tract: the lungs.
What happens in the lungs?
In more serious cases, the lining of the respiratory tree – the passages between the windpipe and the lungs – becomes injured, causing inflammation. More serious still, it can go all the way down the airways to the alveoli, the air sacs in our lungs that send oxygen to the blood and remove carbon dioxide. Then breathing becomes more difficult. As cells die, this causes an outpouring of inflammatory fluid and white blood cells into the lungs, which in turns causes even more damage. This is pneumonia, which can lead to both secondary infections and acute respiratory distress syndrome: lung failure.
How are severe cases treated?
Though drugs are being trialled around the world, there is as yet no specific treatment for Covid-19, so doctors offer “supportive care”: essentially, paracetamol, fluid replacement, oxygen and antibiotics for secondary infections. In the worst cases, the patient needs the help of a mechanical ventilator to continue to breathe. This involves sedating and intubating the patient – putting a tube down their throat – then pumping air into their lungs. Some nonetheless die. How the immune system reacts is apparently the key factor in determining how bad a patient’s condition gets. In some of the most severe infections, the immune response goes haywire, setting off what is called a cytokine storm.
What is a cytokine storm?
Cytokines are proteins used by the immune system as alarm beacons – they recruit immune cells to the site of the infection. The immune cells then kill off the infected tissue in a bid to save the rest of the body. When working properly, this process is confined only to infected areas. But sometimes, the immune response spins out of control and those cells start attacking healthy cells as well as damaged ones. Inflammation spikes, and fluid fills the lung sacs, essentially drowning the patient; the condition can spread into the circulatory system and also spiral into multiple-organ failure. This seems to be what happens in many fatal cases, estimated to be 1% to 3% of infections. The crash typically happens seven days into the disease, leading to a sudden deterioration. The process is not fully understood, but some doctors are treating people who have advanced forms of the disease with powerful anti-inflammatory drugs to combat it.