In 1967, during the first reported filovirus disease outbreak in Europe, the identification of the previously unknown causative agent of the deadly disease was performed by electron microscopy (EM) (Figure 4 ). The unusual filamentous structure of the particles led to some confusion and it was even suggested that the causative agent of the disease might be related to the spiral-shaped Leptospira, a genus of the spirochaetes bacteria [58] . Others concluded that the observed particles were viruses morphologically related to rhabdoviruses and named the newly discovered pathogen Marburg virus [23, 59] . Marburg virions are pleomorphic particles, which appear as rod-or ring-like, crook-or sixshaped, or branched structures. Cryo-EM analysis of purified virions showed that about 30% of viral particles released from infected Vero cells were filamentous, 37% were six-shaped, and 33% were round [60] . The same study revealed a mean particle length of 892 nm and a mean diameter of 91 nm. Previous conventional EM studies showed that the MARV particles were uniformly 80 nm in diameter, whereas the length varied widely with virions measuring up to 14,000 nm. The average particle length was 790 nm [61] [62] [63] . The reported differences in particle size might be due to experimental differences between cryo-EM and conventional EM [60] . Notably, MARV particles are considerably shorter than EBOV virions, although MARV genomes are slightly longer than EBOV genomes [62, 63] . MARV particles are surrounded by a host-derived membrane that is coated with spikes of 5-10 nm in length, which are formed by trimers of the viral glycoprotein (GP) ( Figure 5 ) [60] [61] [62] [63] [64] [65] . The central core of the viral particles is the ribonucleoprotein complex (nucleocapsid) formed by the viral RNA genome and tightly associated nucleocapsid proteins ( Figure 5 ). The nucleocapsids are highly organized tubular structures with an outer diameter of 45-50 nm and an electron-dense central axis of 19-25 nm. The central axis is surrounded by a helical capsid with cross-striations at a 5 nm interval [61] [62] [63] . A recently published detailed cryo-electron tomography analysis of MARV virions has shed some light on the structural organization of the nucleocapsids. Reconstructions of virion-associated nucleocapsids using subtomogram-averaging analysis revealed that the MARV nucleocapsids form a left-handed helix with a pitch of 7.5 nm and a flexible average symmetry of 14.96 protrusions per turn with two inner lobes of density per protrusion. The inner lobes represent the nucleoprotein (NP), suggesting that the MARV nucleocapsid contains an average number of 29.92 NP molecules per turn with each NP molecule packaging six RNA bases [60] . MARV nucleocapsids show directionality, having a pointed and a barbed tip [60] .