Great thread, Gulcher. But how did you know?
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I don't know much about the Spanish flu but will offer some general info.
One of the key factors in a pandemic is the method of transmission. Airborne transmission means the disease floats around in the air. So anyone in the same room (or bus or plane) can be exposed to the disease.
The current virus is spread by droplet transmission. This disease doesn't float around in the air but is does persist for a while outside of the body. So if you get coughed on or touch something that has been coughed on you can be exposed to the disease.
So for an airborne disease one sick person can infect the 200 other people in a plane on one trip. These 200 people can then go and infect more rooms or planes or buses full of people. This can result in millions being infected in a very short time.
The current virus can also spread rapidly but not as rapidly as an airborne disease. One person can cough and spread the virus all around a room. Everyone who touches something in this room that has been coughed on can then get sick and go and spread the disease by coughing on things. Also the room is still contaminated after the first sick person leaves so more people can become sick over a period of time from the same person.
Another key point is the time between a sick person being able to spread the disease and when they show symptoms.
Generally if someone gets sick (shows symptoms) they stay home and go to bed. So even though they can spread the disease it doesn't spread much because they are at home not going near anyone else.
But if they are producing virus and don't feel sick then they will continue to go shopping, go to work, etc. So the disease continues to spread until they get sick enough to stay home.
Also diseases that kill quickly limit the spread. If a disease is fatal in three days then there is only a short time for the sick person to spread the disease.
The current coronavirus is what is called 'novel'. It is new. No-one has ever caught this virus before. So no-one has any pre-existing immunity to it.
Diseases like Spanish flu aren't as big a problem because people have been exposed before and survived. People who were very susceptible have already died. Everyone who survived had some sort of resistance to it. So if it ever blows up again there is an amount of resistance against it already in the population. This can help to slow spread.
So the worst case disease has high mortality, spreads rapidly, persists outside the body, takes a while before people die, lets people spread the disease before they get really sick and is new.
A bit more info for the member who asked about catching diseases through the eyes.
Viruses are very specific. The current virus will bond with human cells which have a suitable bonding site. Cells with this bonding site also occur in dogs which is why you will see reports of dogs testing positive for this virus.
Generally these cells are found in a person's nose and throat. This is why touching your face is a bad idea. If your hands have touched something contaminated by the virus, touching your mouth or nose puts the virus near your nose and throat. Just having the virus on your hands doesn't mean you will catch the disease. The virus can't bond with your skin.
So if a virus can bond with cells in the eyes or travel in the body to cells with a suitable bonding site then; yes, you can catch diseases through the eyes. Generally you will most likely catch them through the mouth or nose because this is a great way for diseases to get inside of you. They have a whole bunch of different cells available to bond with; mouth, nose, throat, lung, intestine, etc.