def ncbi_influenza_genomes_url(database='genomeset'):
    """Construct URL for downloading NCBI influenza genomes database.

    Args:
        database: db to use; 'genomeset' or 'influenza_na'

    Returns:
        str representing download URL
    """
    url = 'ftp://ftp.ncbi.nih.gov/genomes/INFLUENZA/'
    assert database in ['genomeset', 'influenza_na']
    url += database + '.dat.gz'
    return url