The purpose of this study is to see if a drug, called methylprednisolone, is safe and effective in people with Hantavirus infection. Individuals 2 years of age or older are invited to participate in this study if their doctor suspects or knows they have Hantavirus infection. Volunteers will either be given methylprednisolone or placebo (contains no medication) through a needle inserted in a vein for 3 days. During the first 7 days of hospitalization procedures may include blood tests, physical exams, chest x-rays, and urine tests. During study visits on days 14, 28, 84 and 180 after diagnosis, the doctors will ask about health, examine the body, take a chest X-ray, collect blood for safety testing and for measuring antibodies, and do breathing tests on volunteers. Participants will be involved in the study for about 6 months.