The first reason is their size. They are very small and are very difficult to see. Special technology needed to be developed in order to observe them and their behavior and reach the conclusion that they were in fact infectious agents that could cause diseases. This wasn't possible before because the microscopes weren't as well developed.
Another is that they have a very long incubation period. This means that if you have them, a long time will pass before they start harming you which meant that many people believed them to be harmless since they couldn't track their behavior over a longer time period, also due to the lack of good technology.
Answer:
Isolationism was stronger after the depression
Explanation:
After WWI, USA became more active economically and military in aid for Europe's countries in post-war recovery. This meant that many European countries depended on US loans to recover. After the crash in 1929, USA became more conscious about their own economy and stopped participating in European conflicts. This encouraged countries such as Germany or Japan to take action in order to expand territory, which gave place to the raise of fascism in these countries (Japan's INnvasion of China in 1937 and Germany's annexation of Czechoslovakia in 1938, for example)
North and West. USA and Russia. Let me know if that's correct.
