You can infect others without showing symptoms just like with any other virus. It's microbiology 101. There is a growth phase is which virus is being produced and shed that can occur prior to symptoms showing... Not to mention than some of the early symptoms resemble common ailments such as the flu/cold, where people will still go out to work or school without much thought. The only thing that makes this claim possible is if they mean that the virus mostly spread through ways (blood, etc.) that are largely only possible once symptoms appear.