Good question. I think a lot of people are confused.
There are many, many viruses in the world. We don't even know how many. Viruses are not really alive, but they are like living things in some ways. They have either DNA or RNA, the molecule that carries all the recipes to make a new virus. But viruses don't use their own DNA. Instead, they inject it into the cell of a host and turn that cell into a virus factory. All living things can be attacked by viruses. Each type of virus usually only attacks one or a few species, though.
The common flu and influenza are the same thing. BUT, every year new varieties of the flu evolve. You can think of these different varieties sort of like different breeds of dogs. (One obvious difference is that people created different dog breeds on purpose.)
The reason we need a new flu shot every year is because of the changes in the flu virus. Every year there are actually a few new varieties. Experts have to make an educated guess about which one will spread the most. They can't wait to find out because it takes time to produce the new vaccine. Some years the flu is only a problem for babies, elderly people, and people with other illnesses, but sometimes it's deadly for a lot of people. The 1918 flu epidemic was very deadly.
Corona viruses are a different kind of virus than the flu. It's sort of like it's a cat and flu is a dog. There are lots of varieties of corona viruses (like lots of cat breeds). The virus that causes COVID-19 is an especially bad one. It spreads about as easily as the flu, but is several times more likely to cause serious problems.
The symptoms of flu and COVID-19 are similar. That's one reason that public health experts want to make sure people get flu shots this year. The flu shot won't protect us from corona viruses, but fewer people with the flu means fewer people who have symptoms that look like COVID-19. That reduces the need for testing, hospital space, and quarantining for people who just have the flu.
Do you think diseases that spread across the globe are going to be more common or less common in the future?
Thanks for asking.