This is a study in hospitalised adults aged 50 or above with respiratory syncytial virus (RSV). RSV is one of the viruses that cause symptoms like a bad cold, leading to cough, fever, sore throat and a runny nose, that usually lasts for 1 - 2 weeks.

RSV can be severe in older adults, and adults with other medical problems. There are no treatments against RSV or vaccines to prevent RSV that are approved for use in adults, and therefore there is a clear unmet medical need for new treatment options.

ALS-008176 is a new treatment for RSV which is not approved by any regulatory authority in the world, and so can only be used in a research study. The purpose of the study is to find out if ALS-008176 reduces RSV infection, to find out which dose of ALS-008176 is most effective, and to study to safety and tolerability of ALS-008176.