Lassa Fever and Marburg Viruses were the first two to be recognized and caused the scientific community multitudes of headaches until they realized how infectious they were. I remember that Lassa was brought to the US to a lab at Cornell and someone on the next floor up from the research died from the disease. It seemed it was circulated through the air ducts. It goes with logic that airborne particles of any virus could evade immune systems and establish upon mucous membranes thereby infecting the host. The odds are in the virus’ favor. Ebola is in the same family of viruses as Lassa and Marburg.
These filoviruses are extremely hard to grow in a lab as they obliterate the cell cultures rapidly and make them hard to study. Plus, now the labs that work with them have a myriad of safeguards to protect everyone, unlike when I worked with just gloves and a hood.