Here is a very low cost gelatine infected zombie bit prosthetic. It is less likely to cause any skin irritation if you are allergic to rubber latex but still do all the usual tests with the materials to check for allergies prior to application explained in the next step.
The whole process takes about an hour after a couple of attempts and you can re-use your negative mould multiple times as the gelatine washes out easily, as long as your Plaster of Paris mix is strong. Normal ratio is Plaster of paris 2 parts to 1 part water. Simple plastic cups are the best way to measure amounts required.
The most important part of any prosthetic is to ensure you have thin edges for effective blending to the skin. This example is a little rough and needed more blending but it was done just to show the process and less emphasis was put on realism but it's still fairly effective and hope it gives you the basic idea.
Please be aware: Gelatine is extremely hot when melted (like hot toffee) therefore please don't apply it directly to the skin or you may create a permanent special effect prosthetic.