"How real was the threat of Communist spies within the US?"
In 1950, US State Department official Algar Hiss was accused of passing information to the USSR. Hiss denied it, but in 1995 documents were revealed, indicating that Hiss was indeed a Communist spy. Therefore, the Algar Hiss Case in 1950 shows that the threat of Communist spies was real.
It was known that spies had passed helpful information about the atomic bomb to the USSR. In 1951, it was found that two Americans, Julius and Ethel Rosenberg, was responsible for passing the atomic secrets. Thus, this shows that the threat of communist spies in the US was real.
From 1952 to 1954, Joseph McCarthy accused many people for supporting communism. However, his accusations were backed with no evidence, and he never managed to find a communist spy. Therefore, McCarthy's actions, or McCarthyism, is an example of the how the threat of communist spies in the US was unreal.
The threat of Communist spies was unreal because the US government was at fault for increasing the public's fear. Truman's Executive Order 9835 was a program designed to weed out any disloyal persons, but, sadly, it was "in large measure responsible for creating...hysteria" (Zinn 157).
"Is it fair to say that the anxiety caused in the US due to nuclear threat, was useful and 'healthy'?" (security of the nation = health)
The FCDA's Operation Alert, a civil defense drill, was inspired by the heightened state of fear for nuclear attacks. But, most Americans ignored the drill, rendering the FCDA's efforts useless. Therefore, the anxiety caused by nuclear threat was far from useful in the US.
The anxiety caused by nuclear threat was useful and healthy because it led to the growth of businesses in the 1950s. Atomic power fascinated people, and many businesses saw the potential of atomic power, as evident in the production of "fallout shelters", atomic tourism, and "atomic lunch-boxes".