Inclusion Criteria:

1. Age ≥12 years
2. Able to provide informed consent or child assent with parental consent
3. Immunocompromised, as defined by one of the following:
   • Allogeneic hematopoietic cell transplantation (HCT)
   • Lung or lung-heart transplantation
   • Patients treated with chemotherapy for hematologic malignancies or autologous HCT
4. Confirmed Parainfluenza virus by nasopharyngeal swab or tracheal aspirate for one of the following:
   • Respiratory virus panel
   • DFA
   • Qualitative/quantitative RT-PCR test for parainfluenza virus performed at the local laboratory (a confirmatory PCR test will be done at the central lab but is not required to start the patient on study).
5. Female subjects of child-bearing potential who are capable of conception must be: post-menopausal (one year or greater without menses), surgically incapable of childbearing, or practicing two effective methods of birth control. Acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization and hormonal contraception. A female subject must agree to practice two acceptable methods of birth control during the 28 day study period. Abstinence is not an acceptable method of contraception. All reproductive female subjects must have a negative serum pregnancy test during the screening visit.
6. Male subjects must agree to use medically accepted form of contraception during the 28 day study period.

Exclusion Criteria:

1. Psychiatric or cognitive illness or recreational drug/alcohol use that, in the opinion of the principal investigator, would affect patient safety and/or compliance.
2. Any significant finding in the patient's medical history or physical examination that, in the opinion of the investigator, would affect patient safety, ability to use the dry powder inhaler or compliance with the dosing schedule.
3. Subjects currently treated with oral, aerosolized or IV ribavirin
4. Subjects taking any investigational drug used to research or treat PIV