The virus would be the Rodere hemorrhagic fever virus. Rodere is Latin for rodent. Rodents would be able to live their entire lives with the virus without having any symptoms. I chose rodents because they inhabit most of the world and are very populous in forests and large cities. They have also been known to spread disease in the past such as Lyme disease.
The symptoms of this virus would be fever, body aches, vomiting, diarrhea, internal bleeding, and fatigue. Though the virus would have an incubation period of two to three weeks before the symptoms began to show. The virus would be RNA so that it could replicate within a new host with ease. It would be an airborne virus so that it could infect many people very rapidly. People would easily transmit this disease through coughing, sneezing, and vomiting, and because the virus has a two to three week incubation time, many people would be spreading the virus without even realizing they had it. And because the initial symptoms resemble those of the common cold or a much milder virus, people will not think they have a serious illness and will not take immediate precaution, furthering the transmission of the virus.
By the time symptoms begin to appear, the virus will have had time to spread through the body and the person will be very sick. The symptoms will worsen rapidly and if left untreated, the person will die of high fever, dehydration, and internal bleeding within one week. Even if the person were able to get medical help, unless they received it early on, it would probably be too late.
The only way to really stop the virus would be to keep it from spreading. People with it, would need to be quarantined, and everyone else would need to wear masks and avoid large gatherings. There would need to be national travel bans and public places that attract large groups of people would need to be temporarily shut down until the virus runs its course.
References
Quammen, David. Spillover. The Bodley Head Ltd., 2012.
Centers for Disease Control and Prevention. Viral Hemorrhagic Fevers (VHFs). https://www.cdc.gov/vhf/index.html