The lab boss sniffed. 'Extra-terrestrial parasite on a terrestrial host? Ridiculous! The body chemistries would be incompatible.'"
Robert A. Heinlein - The Puppet Masters
Infuriating that of Heinlein's three best known works, two are among his worst written. Along with Starship Troopers, The Puppet Masters recalls the few times he allowed himself to slip from measured, self-aware individualism to rambling chest-thumping jingoism. It dates from 1951, a ripe time for tales of insidious aliens masquerading as real people, thinly disguised morality plays about the subversion of holy capitalism by those damn dirty communists! This was the heyday of the red scare, with Grand Inquisitor McCarthy turning up the heat on the anti-communist paranoia simmering through the previous several decades. That same year, John W. Campbell's 1938 Who Goes There was dredged up and adapted into a movie, before it became "The Thing" years on. It only took a couple of years for the likes of Philip K. Dick and Ray Bradbury to come out with their own alien doppelganger tales (though really, for Dick this sort of paranoia was to be expected.) From then on the "pod people" trope was set in stone as a pop scifi staple.
1951 also happens to be two or three years before the discovery of the structure of DNA. Yet both Campbell in 1938 and Heinlein in 1951 made sure to at least note the implausibility of compatible metabolic/pathogenic processes across planets. The sheer complexity of life in its levels upon levels of organization lends even minute differences the potential for sweeping effects. A single mutation in the nearly four thousand base pairs comprising the hemoglobin gene yields sickle cell, and throws a wrench in the plans of one P. falciparum of sanguine bent. How close a match do you think you'll get between organisms which didn't even evolve using the same genetic code? If an alien parasite found you, it wouldn't know which end to start nibbling.
Science Fiction's golden age came with the resurgence of "hard" SF in the 1940s and 50s. That'd be when Heinlein went as far as to dedicate a couple of paragraphs every other chapter to the arithmetic of interstellar distances... in a young adult novel... and the young adults ate it up! The Puppet Masters is more pulpy, somewhat disjointed, with awkwardly interposed gratuitous sex references and repeated pointless digressions into glorifying the brainwashed murderers employed by the establishment. However, it still paid more attention to the science of its fiction than any of its endless "pod people" copycats through the decades. As after the Verne / Wells era, Science Fiction went slowly into decline, and we are now once again in a 1920s/30s mass-appeal phase. The
I would not want to live in the 1940s but then again neither did most of the people of the '40s. If nothing else, maybe this at least rendered them more capable of looking forward than we are now, still mired in messianic black hat / white hat heroism and narcissistic, myopic social justice tracts masquerading as futurism.