Coronavirus can be present in asymptomatic carriers. Some infectious diseases, like smallpox, have few if any people who are asymptomatic carriers — that is, people who are infected but don’t show any symptoms. The problem with asymptomatic carriers is that they make it very difficult to hunt down the microbe when it can essentially “hide” inside of certain people. The most classic case of an asymptomatic carrier is that of “Typhoid Mary,” who was chronically infected (but symptom-free!) with Salmonella. She was a cook, so everywhere she went, she inadvertently poisoned and killed people.
The same is true of SARS-CoV-2. Far too many people can be asymptomatic carriers, making the virus tough to track down.
Coronavirus can infect many different kinds of animals. Smallpox only infected humans, which is why it was a disease that was relatively easy to eradicate. But microbes that can infect a wide range of animals will be nearly impossible to eradicate. Rabies, for instance, can infect any mammal, which means it will never be eradicated. It simply isn’t possible to track down and vaccinate every raccoon or possum in the world. Similarly, SARS-CoV-2 or similar viruses can infect bats, pangolins, and possibly a whole host of other species, such as primates, rodents, and whales. Recently, we learned that the coronavirus can infect minks.