There are several reasons why Hareidi communities are hit relatively hard by disease outbreaks. Some of those reasons apply more in Israel and less in the US, but that does not really matter since the communities in Israel and the US associate with each other.
The first one is that so many of the activities in the Jewish community require public assembly. Men feel that it a requirement to go to minyan and the bet medrash, and will not forego this in the early stages of the flu, when symptoms are mild (just a little cold) and the risk of contagion is high. Who among us (Hareidi or not) has not participated in such behavior?
Second is the distrust of outsiders. We know that Hareidim vaccinate at far lower rates than does the rest of the population, and one of the reasons (not the only one; see below) is distrust of authority outside of their own communities.
Third is the lack of education. Many in the Hareidi communities have not had an education in the sciences beyond middle-school level, and have no idea of how vaccination works, or why it is safer to vaccinate than not to vaccinate. In many cases, this also applies to the leadership, who the rank-and-file look to for advice. Although well-meaning, it is difficult to offer advice if there is no education providing a foundation for that advice.
Last is the method of dissemination of information in the Hareidi world. In certain communities in Israel, information is disseminated only by word-of-mouth and by pashkevilim. Hareidi society is particularly susceptible to anecdote: “I heard from my neighbor, who heard from the gabbai of a chashuvishe Rav that his niece (chalilah) got very sick from the vaccination….”
Such rumors, in the lack of other information do not make for intelligent decisions, especially about health. Of course, any clinician will tell you that he or she has to deal with these kind of rumors from a great many patients, Hareid or not. However, Hareidim have less access to other types of information, and thus have to give greater weight to anecdotal information.