What does anne frank mean when she says invasion fever is mounting daily?

During the war, other countries came to bomb Nazi-run countries to try to save them. All the members of the Secret Annexe would hear the bomb raid alarms daily and got more and more frightened of an invasion. Each day, Anne would hear talk.... what would we take with us if Holland was invaded? Would we try to swim away? But I can't swim! Well what would we do? We should just stay here and it'll be safer! This would go back and forth and was usually the main topic of conversations. When she says "Invasion fever is mounting daily" she means that people have gotten so conscious about an invasion.