What were the United States' fears about communism after World War II?

As the Cold War between the United States and the Soviet Union intensified after World War II, the very real threat of Communism produced great fears in the American People. There was a real threat that Communists and their sympathizers inside the United States might actively support the Soviet Union and threaten American security. The Soviet Union had a history of conducting espionage inside America with the aid of U.S Citizens. The Rosenbergs, an American couple working for the American Government, actually helped the Soviet Union develop the nuclear bomb.

Internationally, Communism was gaining strength and territory. The Soviet Union tested the nuclear bomb. Mao Zedong and his communist rebels took over China, and in 1950 North Korea attacked South Korea.

Communism was the official ideology of the Soviet Union, and as the Soviets were the opponents of the United States, this inevitably created widespread fear and distrust of communism among Americans. The onset of the Cold War created an atmosphere of paranoia in the United States. Although the number of actual card-carrying Communists was tiny, proponents of the post-war Red Scare painted a picture of a country in which communists and their sympathizers were lurking round every street corner.
The biggest fear among Americans was that communists had infiltrated the US government. This was the basis of the notorious witch-hunts conducted against suspected communists by Senator Joseph McCarthy. Communist infiltration was used to explain what was perceived as the soft policy of containment pursued by the Truman Administration in relation to the Soviet threat. The failure of the administration to pursue a more aggressive strategy in the Korean War was also alleged to be the consequence of communist treachery in government.
According to McCarthy and his supporters, Communists had managed to attain positions of seniority in the upper reaches of the bureaucracy and even the Army, with the express aim of subverting American democracy from within. There was very little substance to these charges, but the general air of anti-communist paranoia meant that such unfounded accusations were widely believed.