def get_header_desc(reader, header_id):
    """Get description field of an header field ID
    
    Args:
        reader (cyvcf2.VCF)
        header_id (str)
    
    Returns:
        str: Information from a vcf header description
    """
    for rec in reader.header_iter():
        d = rec.info()
        if d.get('ID') == header_id:
            return d.get('Description')

    return None