Rabies is nearly 100% fatal because once the virus reaches the brain and central nervous system, it causes irreversible inflammation and nerve damage. The virus hides inside nerve cells, making it nearly impossible for the immune system or most medications to reach and destroy it. By the time symptoms appear — such as confusion, fear of water, or paralysis — the virus has already spread extensively through the brain. At that stage, no antiviral drugs or treatments can stop the progression. This is why post-exposure vaccination before symptoms develop is critical — it gives the immune system time to neutralize the virus before it reaches the nervous system.