I tend to save my worry for things that I have some degree of personal control over. The majority of the things you listed are completely out of the control of human beings, so worrying about them isn't going to alter whether or not they happen. The others that are affected by humans are also threats that I personally have little or no control over. I can spend every hour of every day worried about the threat of nuclear war, but that worry isn't going to change in any way whether or not some idiot with access to nuclear weapons starts a nuclear war.
I wouldn't expect people to sit in a chair as complete nerve wrecks
But I have to say that im actually really surprised of how people look at these things. Either they give the impression that its not real things we are talking about here or that its simply not important enough to care about.
Because personally, I apparently fail to see how any of these, except maybe the volcano one is less of a worry than for instance starving people in Africa are. I can't personally fix starvation there or do anything about poverty around the world. So following some peoples examples here, its not really something to worry about at all, I find that very strange.
Plague
Black Death
From 1331 to 1353. The total number of deaths worldwide is estimated at
75 million people.[16] Eight hundred years after the last outbreak, the plague returned to Europe. Starting in Asia, the disease reached Mediterranean and western Europe in 1348 (possibly from Italian merchants fleeing fighting in Crimea), and
killed an estimated 20 to 30 million Europeans in six years;[17] a third of the total population,[18] and up to a half in the worst-affected urban areas.[19] It was the first of a cycle of European plague epidemics that continued until the 18th century.[20] There were more than 100 plague epidemics in Europe in this period.[21] The disease recurred in England every two to five years from 1361 to 1480.[22]
By the 1370s, England's population was reduced by 50%.[23] The Great Plague of London of 1665–66 was the last major outbreak of the plague in England.
The disease killed approximately 100,000 people, 20% of London's population.[24]
The third plague pandemic started in China in 1855, and spread to India, where
10 million people died.
Antonine
165–180 AD. Possibly smallpox brought to the Italian peninsula by soldiers returning from the Near East; it killed a quarter of those infected, and up to
five million in all.[12] At the height of a second outbreak, the Plague of Cyprian (251–266), which may have been the same disease,
5,000 people a day were said to be dying in Rome.
Justinian
From 541 to 750, was the first recorded outbreak of the bubonic plague. It started in Egypt, and reached Constantinople the following spring, killing (according to the Byzantine chronicler Procopius)
10,000 a day at its height, and perhaps 40% of the city's inhabitants.
The plague went on to eliminate a quarter to a half of the human population that it struck throughout the known world.[13][14] It caused Europe's population to drop by around 50% between 550 AD and 700 AD.[15]
Cholera
Since it became widespread in the 19th century, cholera has killed
tens of millions of people.
Influenza
Asiatic or Russian Flu lasted one year, 1 million death.
Spanish flu lasted around two years and is estimated to have killed between 20 to 100 million people.
Asian Flu lasted one year and killed 1 to 1.5 million
Think about the amount of death from those above and I didn't include all, and this is in societies where its not easy to travel around as fast as it is today.
Vaccine
A vaccine probably would not be available in the initial stages of population infection.[83] A vaccine cannot be developed to protect against a virus which does not exist yet. The avian flu virus H5N1 has the potential to mutate into a pandemic strain, but so do other types of flu virus. Once a potential virus is identified and a vaccine is approved, it normally takes five to six months before the vaccine becomes available.[84]
The capability to produce vaccines varies widely from country to country; only 19 countries are listed as "influenza vaccine manufacturers" according to the World Health Organization.[85] It is estimated that, in a best scenario situation, 750 million doses could be produced each year, whereas it is likely that each individual would need two doses of the vaccine to become immuno-competent. Distribution to and inside countries would probably be problematic.[86] Several countries, however, have well-developed plans for producing large quantities of vaccine. For example, Canadian health authorities say that they are developing the capacity to produce 32 million doses within four months, enough vaccine to inoculate every person in the country.[87]
There are about 7.7 billion people in the world and in a best case scenario its estimated that we can make 750 million a year, divide it by 2 as people might need two shots its 375 millions on a global scale a year.