Set virus infect threshold to 0, and damage ticks to a very large number (like 10000). You could set use ticks to a very large number (like 10000) as well, but this is redundant when the infect threshold is 0.
This will disable immunity depleting over time when immunity is already low, and when immunity is 0 you'll be damaged very slowly.
Immunity will still deplete when inside a deadzone without a gasmask, but you shouldn't die as quickly.