"The rotting, festering Cow Head can be tossed using the primary fire key or laid down gently on the ground using the secondary fire key. How does it work and what does it do? You figure it out, Sherlock." — The diseased cow head's description in the manual
The diseased cow head is one of the most powerful weapons. It's unique compared to others, as it has a completely different usage. When thrown at a victim, it will make a yellow mist spew from the head, causing the victim to panic and then eventually throw up blood and die. The Cow Head itself can kill if it hits a person. However, if the Postal Dude walks into the yellow mist, his health will rapidly decrease. It also has a secondary fire, in which Postal Dude will set the head down, allowing for it to be shot and explode into mist.
Cow heads can be picked up especially from stoves. One special cow head is placed on an altar in the Suburbs, and apparently those who were making offerings died from severe burns. Interestingly enough, a burned cat is the only survivor.
Equipping a diseased cow head and walking near civilians will have them throw different comments against the Postal Dude, such as him being "unsanitary".