Sapper -- as somebody who has a son with a scientific background who lives in Shanghai (and has lived in China for 9 years) who we WeChat to regularly plus the partner of our other son being an A&E doctor, permit me to give you a better insight.
Stranded has outlined the basis very well. The serious issues with this virus is how infectious it is and therefore how difficult it is to inhibit without serious social and economic disruption. Plus the average death rate is x10 that of winter flu. ( 2% .v. 0.2%)
Yes the vast majority of those infected will only have mild symptoms but the two cohorts who are very vulnerable 1) the elderly because of weak immune systems 2) those with cardio, diabetes, asthma or cancer etc can get very ill quickly as this virus attacks their lungs and over-works their hearts, as they struggle to breath. Those with poor circulation can find their production of antibodies to fight the intruder is also compromised. The comfounding factor is many older people have various health issues that can compound the mortality rates. From Chinese data I think the cardio death rate is some 10% and diabetes and asthma around 6%. The other issue not illuminated in the media, is that some of those that have been seriously ill and recover have more permanent pulmonary(lung) damage .
The current thinking here seems to be to try to protect these groups whilst 'herd immunity' is built up in the general population. This seems dangerous, in that you cannot lock away all these people for a year and herd immunity usually evolves/develops over time, even decades as per polio and measles.
Unless we are abandoning our civic duty as a society and becoming full on 'Darwinian' ( survival of the fittest) then maybe the current policy is too casual. The Chief Scientific Officer seems to be an advocate of the herd immunity approach and I can understand the modelling and the concept. But to get 40million os us affected with a base base mortaility rate of 0.5% your model is volunteering for some 200,000 deaths ! This presumes that we can effectively isolate and protect the 2 cohorts in particular danger as this virus rips. I'm not sure the NHS or the care system has the expertise and resources to do that.
This is why Sapper it is a potential nation emergency .