Saliva inside the brain will almost certainly lead to death.
The mouth is one of the dirties parts of the body. We have tremendously powerful systems at play which try to keep the bacteria in check, like a saliva powered slip and slide that leads to a pool of sulfuric acid to take care of anything we don't like. It's the price we pay for wanting to be able to eat things.
The brain, on the other hand, is immunoprivileged. The blood-brain barrier is a tight packing of cells whose primary job is to keep out anything nasty. It also has the side effect of hindering the immune system from getting to the location. This is one of the reasons why brain eating amoebas are so very nasty. Our immune system simply has trouble getting there. It does have ways, and that is a field of current interest for biologists, but there are limits.
Combine this with the well known fact that bites are nasty and tend to lead to dangerous infections faster than we'd like, and its highly likely the bacteria in the brain would be able to do some real damage. Cat bites, in particular, are known for their ability to get infected because they can get saliva past where our mechanical methods of cleaning can get and past where the body's natural methods can deal with. The result of a cat bite is more often than not an abscess which never fully heals. It is left as an exercise for the reader to imagine how bad an abscess in the brain might be.
Of course, there are other options. Consider that the bite of the Komodo Dragon was long considered to be a highly poisonous concoction which resulted in a slow and agonizing death. It's saliva was considered to be just that dangerous. Later study showed that the methodology of the analysis was flawed. The real issue was that the bites were on the leg of the prey animal, such as a cow or a buffalo. That animal typically went back to its home turf, which had standing water. I'll leave it to you to draw your own conclusions as to what might be in the standing water in the home turf of a ruminant like a cow, and how that might apply to particularly deadly choices of material to teleport.
Of course, there is the question of aim. If your teleporter has surgical aim, there are many places where something small could do grave damage.