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Biting living people isn't easy and can be quite a pain for the one who bites. Human jaws are not made to bite stuff while it's still alive.
An easier way to infect other people and turn them into zombies would be projectile vomiting.
But I'm running into a problem, how many times would a zombie be able to use this method before it is forced to refill? Like how many times can it spit vomit before it has to fill its stomach again?
abestrange assumed 30 spits per day, I'll see what I can come up with.
Maximum liquid contents for human stomach: On average, the human stomach holds about 1 liter of fluid. However, the human stomach can stretch to hold about a gallon (slightly over, but not by much). So, it's safe to assume we have a gallon to work with.
As abestrange pointed out, zombies likely have an increased rate of Gastric Acid Regeneration (GAR). Also pointing in their favor is the fact that traditional "puker" or "spitter" zombies have distended stomachs, a clear indicator of increased stomach capacity.
Assuming two cup's worth of vomit for each attack, a gallon's worth gives us eight vomits or spits. This would be for a regular zombie that lacks an overgrown stomach, which I assume would hold a lot more fluid; let's say sixteen gallons. That's 128 spits.
If we go for a liter (4.25 cups per liter is a good estimate, it's really 4.22675, or rounded up, 4.23 cups) instead, that's 3 vomits for a regular zombie and 48 spits for a "bloated spitter."
On a final note, what's wrong with biting or scratching? Biting and scratching can do a surprising amount of damage (not speaking from experience here, but I have a pretty good idea of how damaging those are), and besides that, after your zombies run out of fluid, they're going to need another defense/attack/infection method. Not only would fangs and claws help a lot in making your zombies deadly, but any sane person can and should be freaked out when people are biting and scratching. It's just not natural.
Vomiting in non-undead humans is a reflex that's designed to empty the entire stomach. Basically, it happens when your body thinks you've eaten something that would be harmful to digest like poisonous berries. This means, that without "refills", your zombies could basically only projectile vomit once before they're limited to only hucking up dregs. They would need to eat/drink between attack vomits and the people who become zombified would need to have eaten recently to make sure they've got a full stomach.
It would be far more effective to have the zombies cough, sneeze, or simply exhale contagion if you want to give them a breath-attack. Airborne diseases are even more nasty than contact-transmission ones and having the zombies constantly exhaling zombie-disease would also add some potentially interesting worldbuilding:
The issues you're going to run into with trying to move to a projectile based infection vector is that your exposed target surface area is MUCH smaller. Almost anyone has a strong enough jaw to break skin almost anywhere on another human, even through most clothing.
Spitting at a target is going to require hitting their face, and a large number of commercially available products can stop whatever fluid you are spitting from being effective.
Furthermore, spitting range is only going to be a few feet at best. If your zombies have already closed that distance to their target, why have them stop, take aim, and spit when they could just continue running/ shambling and bite and scratch them?
Humans produce around 1.5 Liters of Gastric Acid daily though, your zombies could double that. Assume each spit is 100mL and you've got 30 spits per day.
Voiding the entire stomach in a single go would probably be wasteful and ineffective, but spitting a solid mouthful a time might be a good balance between volume, range, and accuracy.
Your zombies will vomit once or twice before needing to fill up again, and if there's a second time it won't go that far. You could make them intelligent enough to still use knives, sharp edges, etc instead, open wounds, then spread the virus through saliva. Maybe they bash their victims on debris and rocks till they get open wounds.
