Question: why does diseses spread around the world

Ruth Elderfield answered on 17 Jun 2015:

Diseases that spread in the form of infections (colds, flu, measles etc) have only one real purpose. To keep on spreading, making more copies of themselves. In order to do this they need something to grow on (or in for viruses), they have to find a host (which can be us) to provide the building blocks to make new copies of themselves.
Often they can only survive in use for short periods of time as our bodies fight back (using our immune system) and they are killed off. The same disease can’t attack us again because we have created little floating defences (antibodies) that recognise the intruder and tell the body to kill it quickly.
So before we kill the disease off the first time it attacks us, it needs to make lost of copies that can jump to the next host (person) and attack them. It is that jumping that encourages the virus to spread.
Now a long time ago people didn’t travel as much as they do today. But even then, the small bit of travelling that happened could have a big effect. The Spanish went to South and Central America and took Small Pox with them. The Spanish had mostly been attacked by the virus before, so had the antibodies to fight it off. The guys in America were not do lucky, nobody had antibodies so the virus was able to quickly jump from person to person.
Today we travel a lot more, we carry our diseases around and share them with each other far more frequently.
Then we have birds and animals, they migrate naturally with their diseases and spread them between each other and very occasionally humans. We also ship animals around the globe, spreading diseases that way too.
Because we know this happens, people and animals are often vaccinated so that they can make their own antibodies before they see the disease in another country.
Sometimes animals and people are placed in quarantine, in which they are separated from other people or animals after they have travelled to a place where the disease was around and they may have come into contact with it.