Bats have "peacefully co-existed" with viruses for a long time and should not be blamed for the Covid-19 pandemic, said infectious diseases expert Wang Linfa.
Instead, it was human activities such as wildlife trading and farming that likely caused the outbreak, added Professor Wang, who is director of the emerging infectious diseases programme at Duke-NUS Medical School.
He warned that more bat-borne viruses could make the jump to human hosts in the future if such activities are not stopped.
The professor was speaking on Thursday in the second episode of a webinar series organised by the National University of Singapore (NUS), National University Health System and the World Health Organisation's Global Outbreak Alert and Response Network.
He said the coronavirus responsible for Covid-19 is one of several viruses that likely originated in bats and were transmitted to humans via intermediate animal hosts.
The Hendra virus, the Nipah virus and the Sars (severe acute respiratory syndrome) virus were passed to humans through horses, pigs and civets respectively, he said.
The identity of the animal in between bats and humans in the case of Covid-19 is not yet certain.
Identifying the origin and early transmission of the coronavirus is not a purely academic exercise, Prof Wang noted. "If you don't know the origin and the intermediate host, it's very hard to prevent future outbreaks."
Animal viruses rarely make the jump to human hosts and scientists have been working to learn how such viruses switch hosts and what factors influence such events.
Pangolins have been identified as a possible candidate in the transmission of the coronavirus to humans.
Prof Wang said the coronavirus - officially known as Sars-CoV-2 - is 80 per cent genetically similar to Sars and 96 per cent similar to a known bat coronavirus.
But another coronavirus found in pangolins has a structure called the receptor-binding domain that is even more similar to the Covid-19 virus than the bat coronavirus.
Identifying intermediate hosts can take a long time. It took scientists 10 years to confirm that civets were the intermediate host that transmitted Sars to humans, Prof Wang said, adding that more conclusive evidence is still needed to establish if pangolins played a similar role in the case of Covid-19.
Prof Wang said the field of serology - the study of antibodies in blood serum - is at the forefront of research on intermediate hosts.
Serology also plays a role in other aspects such as diagnosis and epidemiology, he added.
Singapore was a world leader in using serology for contact tracing, he said, referring to the way serology was used to link three clusters of patients here by identifying patients who had already recovered from Covid-19.
Another major question that researchers are exploring is why Covid-19 causes mild or no symptoms in younger patients.
Prof Wang also said Singapore is among several countries working together to investigate this question on a large scale.
Asymptomatic or pre-symptomatic transmission is one reason Covid-19 poses such a formidable challenge for healthcare systems around the world, said infectious diseases specialist Paul Tambyah.
In the first episode of the webinar series on April 9, Prof Tambyah, of the NUS Yong Loo Lin School of Medicine, said the virus can also cause a range of different, non-specific symptoms or no symptoms at all. "It's really hard to tell who has Covid-19 based on the symptoms."
He said a Covid-19 patient he attended to had seen a general practitioner after experiencing mild shortness of breath.
But by the time she was tested for the coronavirus, she was "completely asymptomatic".
Prof Tambyah added that despite not experiencing breathlessness, coughing, sore throat or any other symptoms, the patient continued to test positive and was hospitalised for eight days until she tested negative twice in a row and was discharged.
Addressing reports that loss of one's sense of smell is a tell-tale sign of coronavirus infection, he said a small study done among National University Hospital (NUH) patients found more than half had lost their sense of smell.
One possible biological explanation is that the virus could be entering patients' systems through the respiratory epithelium and olfactory epithelium, which are tissues lining the respiratory tract and nose.
These tissues have cells that feature the ACE-2 receptor, which has been found to be an entry point for the virus, said Prof Tambyah.
"The virus has to get into the cell, otherwise we will just sneeze it out or it'll just go right through us," he added. "It happens that the cells in the olfactory epithelium do have this receptor. So it is possible that's how the virus gets in and damages the sense of smell early."
Another feature of the virus is that, unlike the Sars virus, the viral load does not correlate with the severity of the illness.
Patients with Covid-19 have the highest amount of the virus in their bodies at the onset of the disease when they may not feel sick. In Sars, the viral load tended to peak when patients were at their most ill, Prof Tambyah said.