Why Don’t Viruses Make Their Original Hosts Sick? 5 Questions Answered

The ConversationEditor’s note: The coronavirus that has claimed more than 4,000 lives worldwide and sickened more than 113,000 most likely originated in bats, most experts believe. From bats, the virus “jumped” to another species, likely pangolins, and then to humans. Why didn’t the virus make bats or pangolins sick? As it turns out, viruses are complicated — in addition to sometimes being deadly.

1. How Does This New Virus Differ From Other Coronaviruses?

The family Coronaviridae contains about 39 different species of coronaviruses. Of these, only seven coronaviruses have been reported to infect and cause disease in people. Four coronaviruses cause mild symptoms similar to the common cold, but three coronaviruses cause severe and possibly deadly infections: the severe acute respiratory syndrome coronavirus (SARS-CoV), the Middle East respiratory syndrome coronavirus (MERS-CoV), and now, SARS-CoV2, which is responsible for the current coronavirus disease COVID-19.

SARS-CoV2 is a cousin of the coronavirus that caused SARS, having about 79 percent similarity in its genetic makeup. Though similar, these two viruses are not the same, and their disease manifestations are different. SARS was recognized at the end of February 2003 in China. Worldwide, 8,098 people became sick with SARS and 774 died, with the disease having a mortality rate of 10 percent.

MERS-CoV was first identified in Saudi Arabia in September 2012. Globally, MERS-CoV was responsible for 2,494 MERS cases and 858 deaths, with a mortality rate of 37 percent.

The ongoing SARS-CoV2 epidemic and the rate of infection and mortality seem different than both SARS-CoV and MERS-CoV. As of March 10, the U.S. has 866 COVID-19 cases with 28 deaths, while 45 cases have been reported in Canada. It seems that SARS-CoV2 is less deadly than the other two coronavirus strains, but it is more contagious.

2. Some People Are Saying COVID Might Become Endemic. What Does This Mean?

Aggressive diseases like SARS give rise to epidemics – outbreaks where the number of new cases flares up rapidly in a region. Effective, evidence-based public health measures reduce the number of new patients infected, until these aggressive diseases are controlled. In contrast, an endemic disease is constantly present in a certain geographic region. A good example of an endemic disease is malaria, which is constantly present in tropical regions of Africa, Asia and Latin America.

The 2003 SARS epidemic was controlled by a combination of effective international surveillance methods and local, evidence-based public health measures. International surveillance systems alerted the authorities of the emergence of a novel disease, helping set up guidance for travelers, airlines and crew. It also set in motion a global response that prevented the spread of the disease, and helped the local public health efforts to identify and quarantine infected people. Effectively, this combined response prevented SARS from becoming endemic.

By July 2003, four months from the onset of the outbreak, human-to-human transmission of SARS had stopped.

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