Seems like almost every year we are encountering some sort of serious outbreak in a given region of our world, whether it be a variant of the coronavirus, ebola, the avian flu, or even the black plague (yes it is still lurking out there). In fact, I've read a number accounts of concerns being raised regarding the re-emerging of smallpox. To top all this off we've recently discovered dormant viruses buried in glaciers dating back thousands of years.
WHO (Worldwide Health Organization) has stated many times we are on the verge of a worldwide pandemic. The coronavirus is not, as yet considered one.
Viruses. Why do they even exist other than to cause ongoing suffering? Strangely enough their goal is simple - to multiply at the expense of their host. Not much different then a parasite in a sense.
What makes a really dangerous virus? We'll start with a couple basic terms:
Incubation Period - the time one is infected before exhibiting symptoms.
Contagious/Infectious Period - the time one is shedding the virus and could infect others.
These periods may overlap. One may be contagious and NOT show any symptoms (for a time). This is different for each virus of course, but viruses that exhibit this ability are not uncommon: measles, chickenpox, rubella, and on and on...
Consider a virus with an extended incubation period, where the host experiences no symptoms, and is unknowingly quite contagious to others.
Next point is in regard to mode of transmission, accompanied with the ability of a virus to survive without a host, and lastly, its preferred attack vector - or way of infecting the host.
Blood-born viruses such as dengue-fever, malaria, and many STDs are good examples of diseases that are transmitted through the exchange of blood and/or bodily fluids. You would think these viruses would have a hard time surviving, yet they manage quite successfully year after year.
Raise the danger level up a notch, however, and you now have the common flu/cold, and variants such as H1N1 (swine flu) or H5N1 (avian flu), H1N1 (spanish flu), and of course let's not forget the Covid-19 (coronavirus) with all its similar sub-types (MERS,SARS). These particular nasties attack the body either through breathing it in, or landing near facial entry area including the surface of one's eyes. Their primary method of transmission is traveling through the air in suspended droplets ejected from an infected coughing host. In addition, they are hardy enough to withstand a range of temperature fluctuations, and can remain dormant but alive on surfaces for many hours.
Again this demonstrates the need to wash your hands many times during the day. You pick up a nasty on the escalator rail, then touch your face - too late.
There's a term for "rate of infection" they call r-naught (R0). The higher the number, the worse the infection propagates through a population. Measles is 12-18 (essentially the highest we know of for now). In comparison, the 1918 flu that literally killed millions was only around 2.4 on this index. Note: If you get under 1.0, the infection will usually fizzle out. The R0 for coronavirus Covid-19 is ranging from 2.2 and 3.1 depending on the sources. This essentially means a person who contracts the virus is highly probable to further infect 2-3 others.
Now if a virus was particularly ruthless, and tended to kill its host, the R0 would likely drop, as its infectious period would be shortened. To be truly successful, a virus must be devious, take its time to incubate and infect the host's systems to the point that host is contagious. Upon onset of symptoms the virus will then proceed to make the host truly miserable for an extended period of time (short of not killing the host), and throughout this time actively shed itself, ensuring the host is highly contagious throughout, effectively managing to replicate itself over many, many hosts.
Anyone familiar with this sort of behaviour? Alas, it is the common cold and the dreaded flu.
As eluded to above, many viruses are not benign, and they have an established mortality rate - usually expressed as a percentage. One must consider the population affected to understand this number. If a particular population has little to no previous exposure to a similar virus family, then the rate goes up. If the population is aged, or possibly very young, and their immunity systems are weak, the rate may go up. How healthy and adaptable the population is plays a factor here.
Let's revisit the yearly flu. It usually averages between 1.5 - 1.8%. In contrast, the 1918 flu was estimated to be in a range from 10-20% (although the upper range representing the extreme, and we can't know for sure, as this outbreak was worldwide and not properly documented). The 2019 flu season numbers are still in progress - but we are in the range of multi-millions infected and tens of thousands have died. This is a bad flu season in comparison, regardless of the shadow cast by the coronavirus.
Depending on your information source, the current coronavirus outbreak is ranging from 2.5% to as high as 11%. So why doesn't this outbreak seem as deadly as the 1918 spanish flu outbreak?
I have my theories, but it is very important to understand what often accompanies these lousy infections: Pneumonia (viral or bacterial, doesn't matter much). Once the virus is done with us, we are ripe targets. In fact. organizations such as CDC/WHO often amalgamate the two when they present mortality/morbidity rates for flu strains.
Some good news: we have some things they didn't have back in 1918 - antibiotics and vaccines. Add this to our repertoire of medical practices, plus the general understanding by the average person to avoid exposure in the first place - and things are much better.
Now that we've touched on the basics, and in the process avoided explaining how our immune system actually defeats a viral attack, maybe we should touch on that too.
Our immune system is a collection of different "fighting" cells that are dispersed throughout our body. They use the bloodstream and lymphatic systems as highways to move throughout our body. Most of these cells originate from within our bone marrow starting as stem cells and morphing into T or B (lymphocyte) cells, and many other types as well. The key thing here (and yes I am simplifying) is that the our immune system creates "antibodies" which are soldiers built specifically to defeat a given virus. As each virus is unique in its overall shape/configuration - not all our soldiers can reach the heart of the virus successfully - to infiltrate and kill it. So we have all these very unique antibodies standing ready to be replicated and sent onto the frontline. This is why vaccines work to ready our bodies for a given virus. We inject pieces of the virus (dead, or DNA/RNA strings - let's call them antigens) into our bodies, which then allows our immunity cells to learn and adapt, effectively readying our new soldiers.
And all of this works great as long as the immune system recognizes this antigen as a foreign invader...
Last but not least, as I've saved the best for last (of course)...
There is something they refer to as mutation - meaning that the virus modifies itself during its generation process, and changes itself sufficiently to stop our antibody soldiers in their tracks. We still don't fully understand this mechanism, although we have discovered many viruses are actually internally programmed to mutate. Adding insult to injury - the more successful the virus is in infiltrating many hosts, the greater the odds we will see a mutation occur.
When a mutation occurs all bets are off - the virus can adapt new and even more dangerous abilities.
Mutation is why we have a new flu season almost every year.
Mutation is how a serious outbreak turns into a pandemic.
So to wrap up (since I am a sci-fi writer). Maybe the real danger in us finding our precious "sister world" to Earth will not be competing sentient aliens. Maybe the danger will be the foreign nasties operating at the cellular level. Our act of colonization will result in us fighting a different kind of war.
I think I need to educate myself more on biology...
If you'd like to understand the immune system and how it works in more depth, try this youtube series link, fascinating stuff: https://www.youtube.com/watch?v=_jBpv9fYSU4