A newly discovered ebolavirus, Bundibugyo ebolavirus, was identified as the cause of a large hemorrhagic fever (HF) outbreak in western Uganda in 2007. This virus is genetically distinct, differing by more than 30% at the genome level from all other known ebolavirus species. It is most closely related, albeit distantly, to the Côte d'Ivoire ebolavirus, which was previously found in western Africa. The discovery has important implications for the design of future diagnostic assays to monitor Ebola HF disease in humans and animals, as well as for the development of effective antivirals and vaccines. The outbreak, which occurred in November 2007, resulted in 149 suspected cases and 37 deaths. The virus was identified using a combination of molecular techniques, including random-primed pyrosequencing, which allowed for the rapid development of a molecular detection assay. The entire genome sequence of the virus was completed using traditional primer walking methods. The analysis revealed that the newly discovered virus differs from the four existing ebolavirus species by approximately 32% at the nucleotide level, with significant amino acid differences in key viral proteins. The unique nature of this virus has implications for vaccine development, as cross-protection studies will need to be conducted to assess whether existing vaccines will provide protection against this new strain. Additionally, the virus's lower case fatality rate compared to Zaire and Sudan ebolaviruses suggests that it may have different pathogenicity characteristics. Studies in non-human primates are needed to compare the pathogenicity of these viruses. The discovery highlights the power of molecular detection and characterization tools in identifying new pathogens and underscores the importance of using a combination of techniques for accurate diagnosis and surveillance. The findings also emphasize the need for continued research into the genetic diversity of filoviruses and their potential reservoirs.A newly discovered ebolavirus, Bundibugyo ebolavirus, was identified as the cause of a large hemorrhagic fever (HF) outbreak in western Uganda in 2007. This virus is genetically distinct, differing by more than 30% at the genome level from all other known ebolavirus species. It is most closely related, albeit distantly, to the Côte d'Ivoire ebolavirus, which was previously found in western Africa. The discovery has important implications for the design of future diagnostic assays to monitor Ebola HF disease in humans and animals, as well as for the development of effective antivirals and vaccines. The outbreak, which occurred in November 2007, resulted in 149 suspected cases and 37 deaths. The virus was identified using a combination of molecular techniques, including random-primed pyrosequencing, which allowed for the rapid development of a molecular detection assay. The entire genome sequence of the virus was completed using traditional primer walking methods. The analysis revealed that the newly discovered virus differs from the four existing ebolavirus species by approximately 32% at the nucleotide level, with significant amino acid differences in key viral proteins. The unique nature of this virus has implications for vaccine development, as cross-protection studies will need to be conducted to assess whether existing vaccines will provide protection against this new strain. Additionally, the virus's lower case fatality rate compared to Zaire and Sudan ebolaviruses suggests that it may have different pathogenicity characteristics. Studies in non-human primates are needed to compare the pathogenicity of these viruses. The discovery highlights the power of molecular detection and characterization tools in identifying new pathogens and underscores the importance of using a combination of techniques for accurate diagnosis and surveillance. The findings also emphasize the need for continued research into the genetic diversity of filoviruses and their potential reservoirs.