You must mean "fantastic voyage" from 1966 about "human" scientists and their ship who are shrunk down and injected into the body of a dying person,quite an idea for its time,or anyother time...

My mind bending theory is that there exists a race of microscopic aliens we unknowingly share earths biosphere with and they are the true dominant species of earths biosphere and we obliviously inhale them and their miniature starships,literally breathe them into our bodies without even realizing it,as they explore the hidden microverse of inner space within us,just like we explore the macroverse of outer space and thir species inhabit planets we call protons neutrons and electrons and they blast off from the surfaces of sub atomic particles, just like we do from the sub atomic particle we call earth,a strange symbiosos indeed...