The exact magnification in the microscope at the CCD detector was determined using a calibration grid (Pelco International, Redding, CA). Ebola virus length measurements (n = 2090) were made using the Image J software package [52] using the free hand line tool, and the analyse/measure function. For this analysis only viruses containing a continuous nucleocapsid were measured. Viruses with linked nucleocapsids and empty viruses were omitted. The measurements that were made in image J were then collated, analysed, and plotted, using Microsoft Excel.