There are many causes of failure of the body’s immune system, but thanks to certain lobby groups, one cause is better known than all the others combined. Don’t watch the following video if you are easily upset:
Immunodeficiency can be caused by any chronic illness, not just HIV. Malnutrition, diabetes, cancer, alcoholism/drug abuse, TB, connective tissue diseases and many therapeutic drugs are the commonest causes. It isn’t the immunodeficiency that kills you per se, it is the infection that it allows in that does that.
The soul has an immune system too. As long as it is kept clean and well fed with Sanctifying Grace, then no harm can befall it. Temptations are seen for what they are, and repulsed easily, leaving the will untouched. Too many venial sins, or just one mortal sin however, will allow temptation to get in and who knows where that will end.
Soul health and hygiene really is one’s first priority.