What was the most likely reason that many Americans feared the Soviet Union during the Second Red Scare in the 1950s?

The USA and the USSR had opposite ideologies during the Cold War-The USA favored liberal democracy along with a capitalistic mode of production whereas the USSR advocated popular democracy and a socialistic mode of production.
The early fifties were marked by McCarthyism, MacCarthy was a Wisconsin senator who blamed communist traitors in the army or in government agencies, the fears he triggered were fuelled by the nuclear bomb which the Soviet Union had gained in 1947.
Communism had spread both in China and in Eastern Europe and many people in the USA feared it might conquer the entire planet. The traditions and the ideals of the USA, based on freedom and free enterprise were seen as the exact opposite of the values of communism.

The constant presence of diseases or infectious agents within a given geographic area or population group. it may also refer to the usual prevalence of a given disease with such area or group. it includes holoendemic and hyperendemic diseases. a holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). a hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. so probably family history.