Such risks mean people should continue to take action to avoid being exposed to the coronavirus.

Some people who catch COVID-19 have reported experiencing debilitating symptoms for weeks.

In some cases, these symptoms have continued for more than two months.

Some have categorised these symptoms as a form of chronic fatigue syndrome.

This can involve persistent and disabling fatigue, amid various other symptoms.

It is somewhat of a controversial condition, as not all medical professionals necessarily recognise it or know how to treat it.

Previous research has shown clusters of chronic fatigue syndrome followed other infectious outbreaks, including SARS, Ross River virus and Epstein-Barr virus [glandular fever].

Some who face this kind of ill-health get better relatively quickly, while others experience longer periods of post-viral fatigue.

Manchester University researchers have called for funding to "examine the prevalence of fatigue-related symptoms following COVID-19 infection".

Such research could explore "pragmatic relatively low-cost techniques to treat post-viral fatigue to alleviate symptoms and improve the quality of life".

Asked about this type of condition, University of Newcastle Associate Professor Jay Horvat said he refers to them as "extra-respiratory manifestations".

"There is evidence that there are long-term neurological effects associated with a number of viral infections" said Dr Horvat, also of Hunter Medical Research Institute.

"These extra-respiratory manifestations make it even more important to prevent getting such infections, as some of the consequences are actually worse than the initial infection."

There is increasing evidence from studies worldwide that the lungs of people with moderate to severe cases of COVID-19 are not back to normal, even though they might no longer have the virus.

"At this early stage of the pandemic, we still don't know enough about the long-term effects of COVID-19 and whether infected people - particularly those with more severe infections - will get back to normal," Dr Horvat said.

He added that the longer-term respiratory effects were not fully understood. This included those with "even mild symptoms, let alone the potential for extra-respiratory manifestations".

Dr Horvat has examined the roles that viruses and bacteria play in the development and exacerbation of inflammatory conditions.

"There are certainly other respiratory infections that we know can cause various kinds of injury to the lung that can persist well and truly after the pathogen has gone," Dr Horvat said.

"As far as COVID-19 is concerned, I'm sure there will be a focus on the longer-term effects, in terms of respiratory and extra-respiratory manifestations."

As for COVID-19, he said there was "still not a great handle on how every individual fully clears the virus".

"Such information is important for the design of effective vaccines that induce long-term immune responses that protect against reinfections," he said.

Questions remain about whether people can be reinfected, or whether those that appear to have become reinfected have actually never cleared the virus.

Some patients may no longer show the infection in the upper respiratory tract, but it may be "replicating in the lungs and make its way back to the upper respiratory tract later on".

"Also whether that's live virus or dead virus is among all sorts of questions that we're still trying to come to terms with."

Genetic material from dead virus fragments may cause "false positive" test results in people who have recovered from the disease.

Researchers at Hunter Medical Research Institute and elsewhere have shown that symptoms from viruses and bacterial infections can occur "well and truly after infection and associated inflammation has passed".

Ageing lungs, for example, can cause problems to surface from infections that happened decades earlier.

As such, young people should also be concerned about these risks.

"The long-term consequences of younger people being infected with this coronavirus are unknown," Dr Horvat said.

"Just because you don't have those acute over-the-top symptoms that puts you in hospital, doesn't necessarily mean your lungs won't be affected for a period of time afterwards."

In a similar context to chronic fatigue syndrome, research has linked diseases like asthma, eczema, hay fever and auto-immune disorders to various viral and bacterial infections.

These aren't straightforward problems. Modern medicine doesn't hold all the answers to them.

They involve genetic and environmental factors and dysfunctional interactions between mind and body, including the immune and nervous systems.

One theory is that infections can cause the immune system to go haywire and be stuck in a prolonged loop of hyperactivity, causing needless inflammation.

Some observations in this area of science can be counter-intuitive. With allergies, for example, there is evidence that some infections can protect against them. Other infections are thought to affect the immune system in ways that can cause and trigger allergies.

Nevertheless, research has shown a strong link between asthma and viral infections.

Viruses have been implicated in "the inception of asthma, and are recognised as an important risk factor for asthma exacerbations", Ohio State University research found.

Interestingly, Rutgers University research released this month showed that "asthma does not seem to increase the severity of COVID-19".

Dr Horvat said the jury was still out on whether asthmatics were at increased risk of COVID-19.

"More research is required, especially for patients with severe, poorly controlled asthma," he said.