Inclusion Criteria:
* Subjects will be eligible for entry into the study if a nasopharyngeal swab is positive for PIV, RSV, or hMPV (as a single pathogen or a mixed infection with rhinovirus) by molecular assay by a local laboratory AND subjects must fulfill the following inclusion criteria to be eligible for participation in the study:
1. Subjects with hematologic malignancies (i.e., leukemia, lymphoma, or multiple myeloma) or recipients of an allogeneic or autologous hematopoietic stem cell transplantation for one of the following diagnoses: leukemia, lymphoma, Hodgkin's lymphoma, non-Hodgkin's lymphoma, multiple myeloma, and myelodysplastic and myeloproliferative disorder.
2. Subjects who have undergone active cytotoxic chemotherapy within 6 months or subjects who are on an immunosuppressive therapy (e.g., alemtuzumab, ibrutinib, mycophenolate mofetil, corticosteroids ≥1mg/kg prednisone equivalent).
3. Subjects who are recipients of an allogeneic hematopoietic stem cell transplant (HSCT) must be deemed high risk with an Immunodeficiency Scoring Index (ISI) , of greater or equal to 4.
4. Subjects who are recipients of an autologous HSCT must be within 3 months of the transplant procedure.
5. Subjects must be symptomatic with upper or lower respiratory tract symptoms such as rhinorrhea, sore throat or cough and must be dosed within 6 days from the onset of symptoms.
6. Chest X-ray with a Radiologic Severity Index (RSI) score of 6 or lower.
7. Subjects must have pulse oximetry of hemoglobin saturation ≥ 93% on room air.
8. Spirometry (forced expiratory volume in one second \[FEV1\] and forced vital capacity \[FVC\]) ≥70% of predicted value.
9. Adult (≥ 18 years of age).
10. If female, must be either post-menopausal (one year or greater without menses), surgically sterile, or, for female subjects of child-bearing potential who are capable of conception must be: practicing two effective methods of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. Abstinence is not classified as an effective method of birth control.
11. If female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. A pregnancy test must be negative at the Screening Visit, prior to dosing on Day 1.
12. If male, must be surgically sterile or willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. Abstinence is not classified as an effective method of birth control.
13. Ability to understand and give informed consent.
Exclusion Criteria:
* Subjects will be excluded if they fulfill any of the following exclusion criteria:
1. Patients with a pulse oximetry of hemoglobin saturation less than 93% on room air.
2. Known history of chronic pulmonary disease (e.g., asthma \[including atopic asthma, exercise-induced asthma, or asthma triggered by respiratory infection\], chronic pulmonary disease, pulmonary fibrosis, COPD), pulmonary hypertension, or heart failure.
3. Subjects treated for fungal, viral, or bacterial pneumonia in the previous 30 days.
4. Exposure to any investigational agent (defined as any non-FDA-approved agent) within 30 days, or 5 half-lives of the investigational agent, whichever is longer, prior to the Screening Visit.
5. Allogeneic HSCT recipients with an ISI of 3 or less.
6. Autologous HSCT recipients more than 3 months after the transplant procedure.
7. Patients with a relapsed and/or refractory underlying hematologic malignancy with a life expectancy of less than 2 months.
8. HSCT recipients in the pre-engraftment period.
9. Chest X-ray with an RSI of \>6.
10. Patients documented to be positive for other respiratory viruses (limited to influenza, SARS-CoV-2, adenovirus, or coronavirus) within 7 days prior to the Screening Visit, as determined by local testing (additional screening testing is not required).
11. Clinically significant bacteremia or fungemia within 7 days prior to the Screening Visit that has not been adequately treated, as determined by the Principal Investigator.
12. Any condition which, in the opinion of the Principal Investigator, would prevent full participation in this trial or would interfere with the evaluation of the trial endpoints.
13. Previous exposure to PUL-042 Inhalation Solution.
