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?

There are 200 viruses that can cause common cold. The four classifications of virus that can cause common cold are, in the order of descending proportion, rhinovirus, coronavirus, influenza virus, and adenovirus. This diversity makes it difficult to say anything useful for this group of viruses as a whole. It is also the reason why there is no vaccine protecting against common cold.

COVID-19 stands for coronavirus disease 2019, the virus responsible for COVID-19 is the severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2. SARS-CoV-2 is one of the seven coronavirus strains currently known to infects human.

There are four influenza viruses: A, B, C, and D. Influenza A, B, and C are known to infect human.

Both coronavirus and influenza virus are single strand RNA viruses, which means that they use a single strand of ribonucleic acid (RNA) to store their genetic information. All cellular lives, including human, tree, yeast, and bacteria, use deoxyribonucleic acid (DNA) to store their genetic information. Here is where the similarity ends. The RNA is organized differently in each virus and influenza virus. Influenza virus requires an extra step to translate its genetic information into proteins.

Influenza and other types of flu are on one branch of the virus family tree. The Covid-19 virus is on another branch of the virus family tree. I like this diagram:

virus families.

Influenza and common flu typically refer to the same thing. In terms of similarities between the flu and COVID-19, both are respiratory illnesses caused by a virus, but it is a different virus that causes each. The flu is caused by a virus typically referred to as “the influenza virus,” whereas COVID-19 is caused by SARS-CoV-2. SARS-CoV-2 is part of a broader class of viruses termed “coronaviruses.” Other well-known variants of coronavirus are the common cold, SARS, and MERS.

I believe they are all retroviruses (i.e. viruses made of RNA instead of DNA). However, among viruses, influenza and coronaviruses are not closely-related.

The 'common' flu and influenza are the same thing.