In general people don't make fun of vegans or vegetarians - the huge majority of people have NO interest in what other people eat.
The problem is that the only contact lots of people knowingly have with vegans (or even vegetarians) is via sites like this, and when the make silly, misinformed, ridiculous statements, like you have done, they tend to make all vegans and vegetarians appear foolish ( it's like some people believe all Muslims are terrorists, others believe that all vegans are idiots).
First, no animal ever “goes” extinct - they become extinct. It's very, very unlikely that this would be caused by an animal eating a normal diet: humans are omnivores, omnivores normally eat both meat and plant material.
Eating “dead animals” is, I think, preferable to eating animals that are still alive which is what many omnivores do although really, humans don't usually eat dead animals - they eat meat and other products that are produced from dead animals.
I'm not aware of large numbers of people dying as a result of contracting the human variant “Mad Cow Disease” but in the UK I believe only a few hundred people, over many years have even been diagnosed with it. “Mad Cow Disease” was not caused by humans eating meat. As I understand it, it is thought to have resulted, in cattle, because of bad feeding and husbandry practices. Once present, it could then be spread to humans who ate infected meat.
The other diseases you mention have nothing to do with eating meat and are not food-borne.
Edit, No Ruth, NONE of those diseases originated with people eating meat. You really are very misinformed. How can you be SO wrong?
@ Coach Simon: sometimes it better to say nothing and risk people thinking you're a fool than opening your mouth and leaving them in no doubt at all! You need to buy a dictionary and look up “omnivore” as you obviously don't know what one is!