Passage: Gastroenteritis is associated with many colloquial names, including "Montezuma's revenge", "Delhi belly", "la turista", and "back door sprint", among others. It has played a role in many military campaigns and is believed to be the origin of the term "no guts no glory".Gastroenteritis is the main reason for 3.7 million visits to physicians a year in the United States and 3 million visits in France. In the United States gastroenteritis as a whole is believed to result in costs of US$23 billion per year with that due to rotavirus alone resulting in estimated costs of US$1 billion a year.

Question: How many billions of dollars does the rotavirus cost the United States a year?
1