This study is testing a new medicine called **PUL-042** to see if it can help lessen lung infections in people with certain blood cancers or those who have had a special type of stem cell transplant. The infections come from viruses like parainfluenza virus (PIV), human metapneumovirus (hMPV), or respiratory syncytial virus (RSV). PUL-042 or a fake medicine (placebo) will be given three times over six days. The whole study lasts about **30 days**.

• Participants will be randomly given either PUL-042 or a placebo.
• The study involves **100 participants** across **15 locations**.
• Eligibility includes being 18 or older and having certain medical conditions or recent treatments.

To join, participants must have a positive test for the viruses and meet specific health criteria. The study will check on patients' lung health using chest X-rays and other health checks. Some people can't join if they have other serious lung or heart issues or have taken certain drugs recently. Pregnant women or those planning pregnancy can't join, and all participants must agree to use effective birth control. All participants will need to provide informed consent, which means they understand the study and agree to be a part of it.