You need to skin the meat from a zombie dead body, depending of type of strain it will give you the infection.
Then you need medicine skill in order to craft infected weapons, depends on your level which weapons you can craft.
When selecting the rancid human meat it will let you choose which weapon to combine with or in case of the Biohazard molotov for example, you will need the Wine Bottle with Urine on it, then it will allow you to craft it if you have the materials.

How do I create infected weapons? I dont understand

Changes from v199 to v200:
* Make Sunflower Oil takes 120 seconds, not 240.

Changes from v197 to v199:
* New Recipes: Cook Corn x5, Fry Corn Batch (x3), Make Glass Bottle
* Changed recipes: Boil Egg requires 10 FlOz water, not 20.

Changes from v196 to v197:
* You can now melt down many items in the Forge.
* You can now make a hunting knife.
* You can now make a Toolbox.
* Making a Katana now costs more.
* You can now make Biohazard Cocktail and Infected Pipe Bomb.

Changes from v186 to v196:
* New recipe: Build Steel Chest
* New recipes: Fry Corn, Fry Egg, Make Omelette
* New recipe: Make Biofuel, at a Still.
* New recipes: Unpack Large/Medium Tent.
* Extract Gunpowder From Shotgun Shells yields 1, not 4.
* Make Armor Piercing AR Ammo yields 16, not 8.
* Make Assault Rifle Ammo yields 16, not 8.
* Make Armor Piercing Sniper Ammo yields 12, not 6.
* Make Sniper Rifle Ammo yields 12, not 6.
* Make Pistol Ammo yields 20, not 10.
* Make Shotgun Shells yields 8, not 4.
* Make Flint Arrows yields 20, not 5.
* Make Iron Arrows yields 20, not 5.
* Make Steel Arrows yields 20, not 5.
* Make Beet Sugar yields 50, not 6.
* Make Cookies requires 1 egg, not 2.
* Make gunpowder requires 50 Saltpeter (not 45), and 25 water (not 22.5.)
* Make Molotov Cocktail requires Gasoline, not Vegetable Oil.
* Make Pipe requires steel, not lead.
* Make Sunflower Oil taks 240 seconds (not 30), requires 16 seeds (not 4), and now also requires 50 water.