  The term "interference" has been used to describe any setting in which one
subject's exposure may affect another subject's outcome. We use causal diagrams
to distinguish among three causal mechanisms that give rise to interference.
The first causal mechanism by which interference can operate is a direct causal
effect of one individual's treatment on another individual's outcome; we call
this direct interference. Interference by contagion is present when one
individual's outcome may affect the outcomes of other individuals with whom he
comes into contact. Then giving treatment to the first individual could have an
indirect effect on others through the treated individual's outcome. The third
pathway by which interference may operate is allocational interference.
Treatment in this case allocates individuals to groups; through interactions
within a group, individuals may affect one another's outcomes in any number of
ways. In many settings, more than one type of interference will be present
simultaneously. The causal effects of interest differ according to which types
of interference are present, as do the conditions under which causal effects
are identifiable. Using causal diagrams for interference, we describe these
differences, give criteria for the identification of important causal effects,
and discuss applications to infectious diseases.
