  In this paper we present an iterative method to generate an infinite class of
new nonlocal field theories whose propagators are ghost-free. We first examine
the scalar field case and show that the pole structure of such generalized
propagators possesses the standard two derivative pole and in addition can
contain complex conjugate poles which, however, do not spoil at least tree
level unitarity as the optical theorem is still satisfied. Subsequently, we
define analogous propagators for the fermionic sector which is also devoid of
unhealthy degrees of freedom. As a third case, we apply the same construction
to gravity and define a new set of theories whose graviton propagators around
the Minkowski background are ghost-free. Such a wider class also includes
nonlocal theories previously studied, and Einstein's general relativity as a
peculiar limit. Moreover, we compute the linearized gravitational potential
generated by a static point-like source for several gravitational theories
belonging to this new class and show that the nonlocal nature of gravity
regularizes the singularity at the origin.
