Syncytia is formed by fusion of an infected cells with neighboring cells leading to the formation of multi-nucleate enlarged cells. This event is induced by surface expression of viral fusion protein that are fusogenic directly at the host cell membrane.
Syncytia canonly happen with viruses able to directly fuse at the cellular surface without the need of endocytosis.

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J. Virol. February 1994; 68: 1179?1185
Fusion of intra- and extracellular forms of vaccinia virus with the cell membrane
R. W. Doms, R. Blumenthal, B. Moss
J. Virol. October 1990; 64: 4884?4892
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Nancy L. Cole, Charles Grose
Rev. Med. Virol. August 2003; 13: 207?222
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Ronald M. Iorio, Vanessa R. Melanson, Paul J. Mahon
Future Virol July 1, 2009; 4: 335?351
Characterization of cell-cell fusion mediated by herpes simplex virus 2 glycoproteins gB, gD, gH and gL in transfected cells
M. I. Muggeridge
J. Gen. Virol. August 2000; 81: 2017?2027
Reovirus FAST proteins: virus-encoded cellular fusogens
Marta Ciechonska, Roy Duncan
Trends Microbiol. December 2014; 22: 715?724