If we asked a panel of epidemiologists to imagine a virus optimized for rapid spread globally and high lethality, they'd likely include these characteristics:
1. Highly contagious, with an R0 of 3 or higher.
2. A novel virus, so there's no immunity via previous exposure.
3. Those carrying the pathogen can infect others while asymptomatic, i.e. having no symptoms, for a prolonged period of time, i.e. 14 to 24 days.
4. Some carriers never become ill and so they have no idea they are infecting others.
5. The virus is extremely lethal to vulnerable subpopulations but not so lethal to the entire populace that it kills its hosts before they can transmit the virus to others.
6. The virus can be spread by multiple pathways, including aerosols (droplets from sneezing/coughing), brief contact (with hotel desk clerks, taxi drivers, etc.) and contact with surfaces (credit cards, faucets, door handles, etc.). Ideally, the virus remains active on surfaces for prolonged periods, i.e. 7+ days.
7. Those infected who recover may catch the virus again, as acquired immunity is not 100%.
8. As a result of this and other features, it's difficult to manufacture a vaccine that will reliably protect against infection.
9. The tests designed to detect the virus are inherently limited, as the virus may be present in tissue that isn't being swabbed.
10. The symptoms of the illness are essentially identical with less contagious and lethal flu types, so people who catch the virus may not know they have the novel pathogen.
As you probably know by now, these are all characteristics of Covid-19.