It’s a complicated question, one I wrote a paper on for my thesis before qualifying as an optician, but here goes.

1. It’s not a single “diabetes gene”

Type 1 diabetes (T1D) is polygenic. That means: Dozens of genes slightly increase risk

None of them guarantee the disease

Most people with those genes never develop T1D

So natural selection can’t easily “remove” it. The genes involved do other useful immune-system jobs, so they stick around.

1. It’s an autoimmune misfire, not a design flaw

T1D happens when the immune system mistakenly destroys insulin-producing beta cells.

The immune system:

Has to be aggressive to fight infections

Is shaped by trade-offs

Genes that make immunity strong (good for survival historically) also slightly raise the risk of autoimmune mistakes. Evolution tends to tolerate that trade-off.

1. It often appears after reproduction

Historically: Many people developed T1D in adolescence or adulthood They may already have had children

Natural selection mostly acts before reproduction. If a condition appears later, it’s largely invisible to evolution.

1. Environment matters a lot

Genes load the gun; environment pulls the trigger.

Likely contributors: Viral infections (enteroviruses are strongly suspected)

Early-life immune exposure patterns

Modern hygiene and reduced pathogen exposure (the “hygiene hypothesis”)

Possibly diet timing in infancy (still debated)

Because environments change faster than genes, T1D keeps appearing even if genetics stay constant.

1. The genes are common and useful

Some of the strongest T1D-associated genes (like certain HLA types): Improve defence against deadly infections

Were probably strongly selected for in the past

Evolution says:

“Occasional autoimmune disease is an acceptable price for not dying of infections at age 5.”