I have been thinking, if a pony were to be on Earth, wouldn't she or he get ill with the cold or something? Because they're in a new enviroment and their bodies never experienced illness and bacterias on Earth, so wouldn't they get sick easily?

Just think about it, most of the Incas and Antezs were wiped out by smallpox as it's a disease they've never experienced and it was brought by the Spainards when they came to South America.

Well I'm not talking about a pony getting smallpox, but something less fatal like the common cold.

Tell me, would that be possible?

3058222
Sure, it's possible, but it's more likely that the illness wouldn't be able to cross the species barrier.

3058222 Well, you have to think in terms of zoonosis. AKA, the transmission of a pathogen from one species to another, oftentimes between species that are members of separate biological families entirely. Zoonotic transmission would occur only with significant sustained contact between two separate species, and a only with a highly virulent, mutative, easily transmissible, and adaptable disease.

If your story has limited, short term contact between humans and ponies, I would say that something like a rhinovirus (common cold) that is adapted to survive in a human system would have no effect whatsoever without hundreds of generations of cold virus being exposed to the pony system and given ample time to adapt.

However, that does not exclude equine diseases that are endemic to Earth's horses. Something like equine encephalyomyelitis could conceivably jump from a domesticated horse of Earth to a sapient pony of Equestria. And don't forget about things like West Nile Virus, which affects horses much more virulently than it does humans, and it only takes one little mosquito bite to transmit.

Of course, if you wanted to go down the insanely dark path of zoonotic transmission, you could always introduce rabies into the equation. One hundred percent mortality rate, incurable, and incredibly painful. It's deadly to all mammal species, and though they're generally immune to it, birds and reptiles can become temporary carriers of the virus. Scratch that, I think that one person in all of recorded history has survived advanced stage rabies, and the incredibly experimental treatment left her basically as a vegetable.

3058222 I would think mostly short of breath or extremely hot due to the greenhouse gases, or the various exhaust fumes in the air

3058260 I guess, but let's hope hope nopony encounters

1. A Gold Medusa head [Petrification]

3058511
The temperature change due to anthropogenic greenhouse gasses is an entire one degree centigrade thus far. At most. Big deal for climate, not so big deal a deal for individuals on the ground.

Smog would certainly have an effect, but I don't see why it would have more of an effect on a Pony than on a human being. It's very much a health hazard to those in China or anywhere else that fumes are released freely into the atmosphere.

3059290 the heat depends on a region and time

3058339
You just gave me ideas, thank you.

3058339 Holy damn, I've taken note of species difference in my thoughts. I wanted to have one of the character in my story to get a cold one day.