Suicide Bag (自決袋, Jiketsu-bukuro) are small bombs given to those bitten by the Kabane, to commit suicide before they turn into one themselves.
When bitten by a Kabane the infection rate that turns a human into Kabane usually varies for individuals. This can be seen when Ikoma was bitten and the virus started spreading instantaneously, while the corpse of Ikoma's sister only turned after some time. It is implied that the Kabane "virus" spreads through blood circulation (Meaning live humans turn to Kabane at a much faster rate).
A cloth bag with a metallic bottom. A rope with a metal ring coming out of it.
A soldier will take the suicide bag and place the flat part on his chest where his heart is. He will then pull on the rope which will cause a directed explosion originating from the flat part. This will make a fist sized hole where the soldier's heart was, killing him instantly and making him unable to turn into a Kabane due to the absence of circulation.
Using the bag is considered a honorable deed.
If a soldier refuses to use the suicide bag he will be shot in the heart by his fellow companions.
It is speculated that the usage of suicide bags is due to the fact that kabane virus spreads through blood circulation and killing the heart would be the most efficient way of stopping the virus from spreading to the whole body and making the corpse less dangerous to handle.
It can also be used as type grenade, as seen when Mumei threw them to the Kabane.
Another theory suggests that the suicide bags are the "clean" way of executing infected humans without the danger of spilling too much blood inside the trains opposed to the traditional japanese way of executions of seppuku/harakiri.