12. The cartoonist uses the lexical field of disease (with elements such as "fever," "mumps," "pox," and the suffix "-itis" which indicates an inflammation) to represent the wave of totalitarianism spreading throughout Europe. For example, "Hitleritis" and the "Nazi fever" refer to Hitler's dictatorship in Germany, "Stalinitch" is Soviet Russia, The "Italian mumps" is Mussolini's fascist regime in Italy, etc. The U.S. sees itself as the defender of democracy, which is why it views these regimes as a sickness.
13. Uncle Sam being in a separate bed is a metaphor for America being on a separate continent. Even though the ocean stands between Europe and the U.S., the U.S. was not entirely safe from the spread of communism and fascism because people, like germs, can travel and enter other bodies. In fact, in the 1940s, people suspected of being a Communist were persectuted in the U.S.—this is known as the second Red Scare or McCarthyism.
14. The cartoonist is critiquing the feeling among Americans that geography might allow (and even maybe require) them not to intervene in European problems, just like they first thought during World War I. By drawing the beds so close to each other, Dr. Seuss is showing how naïve Uncle Sam is when he thinks he will be able to stay out of this. The idea is that the U.S. will get involved, even if it means that they have to be contaminated for it to happen.