The chemokine receptor gene CCR5 plays an important role in many immune-related processes. Delta 32 rs333, designating the CCR5-delta32 deletion of 32 nucleotides from within the gene, is perhaps the most famous allele of CCR5. 23andMe tests for this by the name I3003626.
Individuals carrying one copy of the delta 32 allele are somewhat resistant to infection by HIV, the virus that causes AIDS, and individuals with 2 copies (delta 32 homozygotes, ~1% of Caucasians) are almost completely immune to infection by HIV. [PMID 8898752] The delta 32 allele may have been selected for in European populations because it confers resistance to plague (Black Death) or smallpox. 
Does the CCR5-delta32 mutation have an entirely positive/protective role?
Probably not. In patients with abdominal aortic aneurysm (AAA), the major risk is a sudden rupture - which is quite often fatal. Individuals with the delta 32 variant are more likely to have aneurysms than non-carriers, and among patients with aneurysms, delta 32 carriers are more likely to rupture than to be diagnosed in time for surgical repair. [PMID 15557916]
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|qualified_impact||Low clinical importance, protective|
|summary||Also known as CCR5-delta32, this variant is associated with resistance to many strains of HIV (but not all strains, only strains that use target the CCR5 protein). Heterozygotes are reported to have slower HIV progression, and homozygotes are very resistant to being infected by these strains.|