There are two important parameters we need to understand the risk caused by any communicable disease. I'm going to broadly say transmissibility, and the probability that exposure will lead to serious disease.
We often see transmissibility represented as a single number, called R0 or "R naught." That's supposed to mean the average number of people who will catch the disease from a single infectious individual, in an exposure naive population. So, as should be intuitively obvious, if R0 is greater than 1, the disease can spread, and if it's less than 1, it should peter out. If R0>1, then the growth in cases will presumably be exponential, although how fast the exponentiation happens obviously depends on how long it take to get to the R0 number. If R0=2 and the person infects those two people in one day, then that person will generate 128 cases in 1 week and more than 16,000 cases in two weeks. If R0=3, then we're talking 4.8 million cases in two weeks. Yikes! (In reality, of course, that number is likely to be spread out over a week or two, not a single day, so this is just by way of illustration.)
Fortunately, that isn't actually true. The number isn't really real. It all depends on the mode of transmission and the social context in which that occurs. Take the case of HIV. The R0 number for any given infected person will be completely different from that of another. It all depends on their behavior, and the behavior of people they come into contact with. It will also vary over the course of their infection. In a properly managed epidemic, it can be held very close to zero; but in some circumstances one person might infect dozens.
Similarly, we are often told that Ebola virus is highly transmissible but that's equally misleading. Transmission requires contact with bodily fluids of an infected person so it's 100% controllable if people follow proper procedures.
The bad news about the SARS2 virus (i.e. the virus that causes CoVid-19, the one that's going around now) is that the mode of transmission appears to be exhaled aerosols and probably fomites, i.e. viruses on surfaces touched by infected people. Even worse news, it appears that asymptomatic people can spread it. That makes it very difficult to control transmission and right now it's looking like it will likely turn out to be impossible in terms of the big picture. MERS and SARS1 were less readily transmissible, so it was possible to snuff out those outbreaks. This one is loose in the wild, there are numerous epicenters of infection including very likely many we don't even know about yet. No meaningful control will be possible in Afghanistan, Iraq or Syria and those countries are almost certain to see epidemics coming from Iran and Kuwait. It's likely that it will turn out to be impossible to contain in countries with strong stable governments and advanced public health and health care systems as early experience in South Korea and Italy is starting to suggest.
This is why CDC is warning us that a substantial epidemic is likely in the U.S. There's just no way to stop it, the virus spreads like influenza but there's no vaccine. I'm not saying that's 100% going to happen, but it's reasonable to plan for it. However, again, that R0 number is not fixed, it depends on how societies respond. Here's a good discussion from The Atlantic.
The good news, however, is that second parameter: how likely is exposure to lead to serious disease. This is why you should chill. The vast majority of cases are mild, or completely asymptomatic. The likelihood of severe disease seems to be similar to that of influenza, although less severe disease is usually less unpleasant than ordinary influenza, i.e. more like a common cold. But it is dangerous for vulnerable people. Here's a good discussion from The Scientist.
The latest data from China stem from an analysis of nearly 45,000 confirmed cases, and on the whole suggest that the people most likely to develop severe forms of COVID-19 are those with pre-existing illnesses and the elderly.So this is not the Black Death. If you aren't in one of those vulnerable categories, you don't have a whole lot to worry about personally. Furthermore, this isn't going to devastate the work force or anything like that. Of course, if you or somebody you care about is more vulnerable, you're more worried, but most of those people can be protected during a local outbreak by staying home, or keeping people at risk out of nursing homes, assisted living facilities and senior housing, and so on. Still worrisome, but literally no more so than influenza, which we already accept.
While less than 1 percent of people who were otherwise healthy died from the disease, the fatality rate for people with cardiovascular disease was 10.5 percent. That figure was 7.3 percent for diabetes patients and around 6 percent for those with chronic respiratory disease, hypertension, or cancer.
While overall, 2.3 percent of known cases proved fatal—which many experts say is likely an overestimate of the mortality rate, given that many mild cases might go undiagnosed—patients 80 years or older were most at risk, with 14.8 percent of them dying. Deaths occurred in every age group except in children under the age of nine, and, generally speaking, “we see relatively few cases among children,” World Health Organization Director General Tedros Adhanom Ghebreyesus said last week.
But, people naturally are going to demand that governments take action, and they are also going to act to protect themselves in ways that aren't necessarily entirely rational from a quantitative risk/benefit point of view. That's why the risk to the U.S. and global economy is very substantial: not because of the direct effect of the virus but because it will stop international trade and travel, squash the travel and hospitality industries, result in school closures, keep people out of shopping centers, concerts, sporting events. These may be overreactions that aren't worth it, but they will happen anyway. So in that sense this is a bigger deal than it needs to be. There could be shortages of medicines and medical supplies due to disruption of global commerce (a lot of medications come from India, for example), lost income results in worse health by various mechanisms, and of course civic order could be strained in many places.
So we're probably going to end up making this a lot worse than it has to be. Don't over-react. There's no sign of community transmission in the U.S. yet, so for now, just ignore it and go about your business. The best way to control infection is frequent hand washing, by the way, and you should do that anyway because of whatever bugs are going around. Face masks don't do much good. If we do hear of community transmission in the U.S., but it's nowhere near you, continue to go about your ordinary business and stay calm. If there is an outbreak near you, and you are in a vulnerable group, you'll probably want to stay home and be careful about who visits you. Otherwise, go about your business until and unless the authorities, or your employer, tell you not to, but don't worry too much.
If I do get sick, but I have mild symptoms and don't have any trouble breathing, I'm not going to go to the ER, I'll let them take care of people who really need it. On the other hand, I definitely will stay home, as you should any time you have a potentially transmissible disease. And employers should encourage you to do so, and not penalize people for it. If I do have trouble breathing, I will go to the ER. And meanwhile, don't stop paying attention to other matters of public concern.