Bats are plentiful, making up as much as 20% of all mammals on Earth. On top of this, they are really good at making sure that the environment stays healthy. For instance, they help to pollinate plants, disperse seeds, and even reduce pesky bug populations.
Many people like them due to the fact that many bats can echolocate, and are the only mammal known to have true flight. Many more people are afriad of them because they think bats suck blood (most bats do not suck blood, they eat fruit). But they also have a dark side as well, they are really good at carrying diseases from one host to another, especially when it comes to transferring from one host into humans, also known as a Zoonotic Disease.
The reason why they are so prone to viruses has a lot of theories behind it. For instance, bats have very good immune systems due to their body's ability to repair broken DNA and reduce inflammation. But this can cause viruses to potentially remain dormant, or work harder to infect the host. As a result, when the virus gets to us humans, with far weaker immune systems, the virus can flourish.
Another reason has to do with the way that bats intermingle. They roost very close to each other in caves, even to the point of crawling over each other. This causes more spread of diseases and viruses, and the fact that bats spew snot and spit when they echolocate (the ones that can, not all bats can echolocate) does not help.
So while bats are awesome and beautiful creatures, you probably do not want to snuggle up next to one.
Check out this recent video on SciShow to learn more about bats and their ability to transmit diseases.