  We construct a vertex operator realization for the simple current primary
fields of WZW theories which are based on simply laced affine Lie algebras g.
This is achieved by employing an embedding of the integrable highest weight
modules of g into the Fock space for a bosonic string compactified on the
weight lattice of g. Our vertex operators are universal in the sense that a
single expression for the vertex operator holds simultaneously for all positive
integral values of the level of g.
