Theres a critical point, somewhere around the second week of coronavirus symptoms, where patients usually start to recover. Sometimes, though, they dont. Some people can take a turn for the worse even after theyve started to feel somewhat better. The progression of symptoms is one of the more mysterious elements of this illness.

While the symptoms can escalate quickly, the more prolonged course of COVID-19 makes it unlike influenza or other similar respiratory illnesses, said William Schaffner, a professor of medicine in the Division of Infectious Diseases at the Vanderbilt University School of Medicine.

Once on a ventilator, patients also seem to be on those ventilators for longer than comparable patients who had the flu and were also seriously ill, he said.

Why do patients who appear better suddenly get worse? And who is more susceptible to this longer progression of symptoms?

COVID-19s Path Through The Body

To understand why some people recover at a better rate than others, we first need to understand the mechanics of how coronavirus infects the body.

As you probably know, the novel coronavirus is a respiratory virus. When an infected person coughs, sneezes or touches a surface, another person can pick up the virus when they inhale respiratory droplets or come into contact with a surface where those droplets have landed, according to Kirsten Hokeness , professor and chair of the Department of Science and Technology at Bryant University and an expert in immunology.

Once in your nose or mouth, the virus then latches on to cells that express a protein called ACE-2; these cells can be located deep in the back of the throat and high up in the nasal passage.

This is an important receptor in regulating blood pressure that the spikes on the surface of the virus have found a way to unlock, Hokeness said. Once the virus unlocks the receptor, it can get inside the cell and use its machinery to replicate, she added.

Once it makes its many copies, the new viruses then exit that cell and go on to infect more and more cells, she said. In some cases, the virus may stay more localized here, and in other cases, the virus replicates and then makes its way down into the lungs and starts to infect the lung cells, as well.

On the way, as the virus is multiplying, it nestles into the lining of the bronchial tubes, causing inflammation and producing cough, Schaffner said.

In the meantime, the bodys inflammatory response is revving up, becoming more active in trying to combat the virus, he said. We get a sore throat, cough, and fever as a manifestation of that.

A cough is meant to expel mucus; a fever slows the path of the virus down, making replication slower. This is still on the milder end of the spectrum, Schaffner said.

After a week to 10 days of fighting it out with the virus, Hokeness said many peoples immune systems will be able to overcome it.

This is enough time for the innate and adaptive immune responses to do their job, subside and develop the memory component of the response, she said, meaning they develop at least short-term immunity.

If, however, the battle rages on and the virus becomes a vicious competitor that the immune system has difficulty controlling, the virus can continue to spread and involve more of the lungs, Hokeness said.

When the virus exits the bronchial tubes and heads to the lungs, it can disrupt the function of the lung tissue, causing pneumonia, Schaffner added. Thats usually a sign that the virus is no longer considered mild.

People can still get better when this happens, but it often takes longer. They may start to decline in week two if the virus does not subside even if, at some points, they seem to feel like theyre recovering. And its not always the virus to blame.