Some viruses are transmitted via secreted body fluids. If these viruses cause people to cough or sneeze, they are much more likely to spread.
If you come into contact with a viral meme, you might want to share it (sneeze or cough) or you might ignore it (asymptomatic).
Viral cat videos are basically only shared by people who enjoyed the viral cat videos. They have a relatively short infectious period (cough/sneeze for a few hours), but the really good ones are fairly contagious. People might see the video, think it’s cute, immediately share it with a few people, and not think about it much after that.
The most viral in-group / out-group, social, political, religious memes have long infectious periods, make people who agree with the message symptomatic, and, crucially, make people who disagree with the message symptomatic. This can only be accomplished by memes that distort, exaggerate, and simplify. Factual, nuanced messages cannot have the same impact.
The freedom to exaggerate and simplify gives the writers of the memes the ability to place boundaries between groups wherever they want — highlighting perceived differences between groups by exaggerating them. The group that the meme favors bonds with each other – “we are so good and they are so evil and this is the ultimate proof of that.” Thus, the in-group gets infected and wants to sneeze in the faces of their tribal members.
The distortion in the message trolls the out-group. With cat videos, people who don’t like cat videos have little interest in sharing them. However, emotionally-charged political memes that are unfair to the other tribe infects the other tribe. If people are making your tribe look bad by lying about it, you will want to debunk it. You end up coughing on your tribe in the form of “look at the outrageous lies that the other tribe tells. They have bad character and are ignorant and this is the ultimate proof of that.”
A good example of these elements is in the trailer to the movie God’s Not Dead. It plays to the in-group by arguing that ignorant atheist professors are trying to stop good, Christian students from believing in God – not because the professor is wiser, but because the professor is angry with God for something bad that happened. It hits on all of the elements of us against them. It trolls atheists by seriously straw manning their arguments.
Simple messages that are only loosely based on facts that distort differences between groups make for very fit memes. They lead to bonding behavior (which is coughing/sneezing) within the tribe that it favors and within the enemy tribe. On the other hand, complicated policy analysis with a lot of nuance, caveats, and questioning of assumptions is hard to remember, lacks emotion-triggering language and images, and weakens perceived boundaries between groups. It doesn’t make people symptomatic. They can be used as medicine or vaccines against the outrage porn memes, but they are expensive and difficult to distribute to the infected populations.