There was "The Dress." And Grumpy Cat. And the Harlem Shake. And some might say there are hints of it in Donald Trump's presidential campaign.

Social media is filled with images, videos and discussions that grab hold of your mind and settle in like unwanted houseguests. Once you've seen or heard them, you can't shake them — and rarely can you keep them to yourself, as you click "share" or start a discussion about their merits.

Some people describe them as "thought viruses," with the belief that these ideas can infect your brain and emotions. Just like going out in public exposes you to cold and flu viruses, when you log into social media, you subject yourself to everyone's thought germs.

Of course, this is hardly new.

Evolutionary biologist Richard Dawkins used the term "meme" in his 1976 book "The Selfish Gene" as kind of a mental equivalent to the gene. He gave examples of memes as catchphrases, fashions, ideas, tunes and even ways of making pots or building arches.

"Just as genes propagate themselves in the gene pool by leaping from body to body via sperm or eggs, so memes propagate themselves in the meme pool by leaping from brain to brain via a process which, in the broad sense, can be called imitation," Dawkins wrote. "If a scientist hears, or reads about, a good idea, he passes it on to his colleagues and students. He mentions it in his articles and his lectures. If the idea catches on, it can be said to propagate itself, spreading from brain to brain."

Dawkins didn't explain how the spread of memes controlled human behavior because he was focusing on genetics. But the meme seed was planted. (Maybe the idea of a meme was actually a meme itself?)

How a meme survives

In his "Memetic Lexicon," memeticist Glenn Grant describes a meme as "a contagious information pattern that replicates by parasitically infecting human minds and altering their behavior, causing them to propagate the pattern." Grant points out that an idea is not a meme "until it causes someone to replicate it, to repeat it to someone else."

So basically if you have this amazing original thought, but keep it to yourself, it's not a meme. Your idea isn't contagious until you put it out there so others can pick it up and pass it around — just like germs lurking on the doorknob of a preschool classroom.

"A meme survives in the world because people pass it on to other people, either vertically to the next generation, or horizontally to our fellows," agrees electrical engineer and memeticist Keith Henson. "This process is analogous to the way willow genes cause willow trees to spread them, or perhaps closer to the way cold viruses make us sneeze and spread them."

Some memes catch on immediately, spread like wildfire, and then fizzle out. Others are slow to build up traction, but then they linger.

"Memes, like genes, vary in their fitness to survive in the environment of human intellect. Some reproduce like bunnies, but are very short-lived (fashions), while others are slow to reproduce, but hang around for eons (religions, perhaps?)" says Lee Borkman of the School of Philosophy in the University of New South Wales, Australia.

More than just cat videos

Thought viruses can be a lot more serious than the latest video of a kitty playing the piano or a "what-color-is-this" discussion.

Political and religious debates can go viral this way, and the argument can get more heated and inflamed as they spread.

Memes are what propels so many people to embrace movements such as nazism, and what keeps racist stereotypes alive, according to Robert Downes in Northern Express Weekly.

"Hitler, with his 'flaming javelin,' was a meme-spreader of the first order. He himself was infected by notions of ancient Teutonic gods, astrology and Germanic myths," writes Downes.

Just out of the Army, Hitler joined the German Worker's Party and was unimpressed with the group.

"But he used his memetic powers to shape the party into one of the most powerful forces in the world," he writes. "As the years went on, Hitler's electrifying delivery held people spellbound at scores of speeches in beerhalls and stadiums, sometimes with 6,000 Nazi storm troopers standing at attention. His message hurled a fiery javelin into the hungry minds of the German people."

How does it work?

You can't choose which thought germs are going to make an impact and which ones will just peter out. But people who are really into this mental viral contagion are fascinated by how altering memes can make them supermemes and how anger can play a major role in meme propagation.

This clever video takes a deeper look. (Don't worry — there are no pouting babies or catchy songs that will stick in your head for days.)

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.