  The realization of a nonlocal quantum field theory without losing unitarity,
gauge invariance and causality is investigated. It is commonly retained that
such a formulation is possible at tree level, but at quantum level acausality
reappears at one loop. We suggest the the problem of acausality is, in a broad
sense, similar to the one about anomalies in quantum field theory. By virtue of
this analogy, we suggest that acausal diagrams resulting from the fermionic
sector and the bosonic one might cancel each other, with a suitable content of
fields and suitable symmetries. As a simple example, we show how supersymmetry
can alleviate this problem in a simple and elegant way, i.e., by leading to
exact cancellations of harmful diagrams, to all orders of perturbation theory,
in the case of a nonlocal Wess-Zumino model. On the other hand, the same is
true for a super Yang-Mills model, but in this case, other important acausal
diagrams are also originating from supersymmetric D-terms. As a consequence, we
conjecture that, for a generic gauge theory, an infinite number of superfields
are needed in order to obtain a complete cancellation between all diagrams.
This framework is naturally occurring in string theory, where infinite
Kaluza-Klein towers of higher-spin fields are generally predicted.
