Exosomes are extracellular vesicles released upon fusion of multivesicular 
bodies(MVBs) with the cellular plasma membrane. They originate as intraluminal 
vesicles (ILVs) during the process of MVB formation. Exosomes were shown to 
contain selectively sorted functional proteins, lipids, and RNAs, mediating 
cell-to-cell communications and hence playing a role in the physiology of the 
healthy and diseased organism. Challenges in the field include the 
identification of mechanisms sustaining packaging of membrane-bound and soluble 
material to these vesicles and the understanding of the underlying processes 
directing MVBs for degradation or fusion with the plasma membrane. The 
investigation into the formation and roles of exosomes in viral infection is in 
its early years. Although still controversial, exosomes can, in principle, 
incorporate any functional factor, provided they have an appropriate sorting 
signal, and thus are prone to viral exploitation.This review initially focuses 
on the composition and biogenesis of exosomes. It then explores the regulatory 
mechanisms underlying their biogenesis. Exosomes are part of the endocytic 
system,which is tightly regulated and able to respond to several stimuli that 
lead to alterations in the composition of its sub-compartments. We discuss the 
current knowledge of how these changes affect exosomal release. We then 
summarize how different viruses exploit specific proteins of endocytic 
sub-compartments and speculate that it could interfere with exosome function, 
although no direct link between viral usage of the endocytic system and exosome 
release has yet been reported. Many recent reports have ascribed functions to 
exosomes released from cells infected with a variety of animal viruses, 
including viral spread, host immunity, and manipulation of the microenvironment, 
which are discussed. Given the ever-growing roles and importance of exosomes in 
viral infections, understanding what regulates their composition and levels, and 
defining their functions will ultimately provide additional insights into the 
virulence and persistence of infections.