[nickles]

This seems similar to an old technique, 'inoculation', which predates vaccination and was used with some success to protect against smallpox.

[0] https://en.wikipedia.org/wiki/Inoculation

[maxerickson]

It's like horse serum. It isn't a less deadly infection conferring the benefit, it is the immune system components of the blood, the antibodies.

https://www.historyofvaccines.org/content/horse-serum