From TF2 Wiki
The Medkit (also known as Bioweapon or Medpack) is the Medic's primary weapon in Team Fortress Classic. It has both offensive and healing capabilities.
When used on allies, the Medkit will fully heal the target. If the target has 100% of their health points, each strike will Template:Cl the target by five points, up to a maximum of 50 bonus health. Adrenalised health will decay over time.
When used on enemies, it will infect the target with a disease. This disease will drain the victim's health until the victim dies or is healed by a Medic. It can also be spread among teammates by contact.
9 every time the Medkit hits the enemy, 8 per infection tic.
Quake Team Fortress
There are a few differences between the Quake Team Fortress Bioweapon and Team Fortress Classic Medkit; the QTF Bioweapon uses Health pickups as ammo and must be switched between “healing” and “weapon” modes by hitting the “1” key if playing on a LAN, and can actually infect teammates. In QuakeWorld TF, player still need to pickup health to use it as a medikit, but does not need to switch between the modes and thus function identical to the TFC variant. In TFC, the Bioweapon does not need ammo in order to be used and does not need to be switched between attack modes no matter how TFC is played.