Raw chicken is generally considered not safe to eat. A lot of people eat sushi/sashimi though, and they are made from raw fish. So, how come that's safe (if it is)?
I discussed a similar subject in this question: What exactly is "Sushi Grade" fish?
Raw fish isn't safe to eat if it's just been sitting around. However, the raw fish used in sushi/sashimi has been frozen (typically flash-frozen) in order to kill any parasites, making it as safe as any other food.
Most distributors of sashimi also have their own methods and internal regulations to ensure food safety, but specifics are understandably hard to come by. Suffice it to say, it's far safer to eat properly-prepared nigiri or sashimi than it is to eat an undercooked hamburger.
As mentioned in this question about raw chicken - some people and cultures do indeed eat raw chicken.
Notwithstanding that, to answer your question more directly, the main problem with eating raw meat, fish, or anything else, are bacteria, parasites and other pathogens.
A healthy animal, however, butchered appropriately, should have no specific issues. As such, we make beef tartare, sashimi (sushi refers to the rice, not the raw fish) and just enjoy!
Sushi isn't 100% safe, but it's reasonably safe (I eat it all the time).
Different animals can harbor different diseases. Chicken are known to carry salmonella, which is pretty harmful to humans. Compare this to most types of sushi grade fish, which don't carry diseases as harmful.
This is also a matter of preparation. "Sushi Grade" fish is prepared very carefully with raw consumption in mind. If a chicken is raised guaranteed free of salmonella, or care is taken to make sure the muscle meat never came into contact with organs or feathers, it would be safe to eat raw. However, the fact is that almost no chicken meets that criteria as there seems to be no demand for raw chicken.
I once watched a TV program in which a parasitologist was interviewed. Many kinds of meat and fish contain parasites that can cause harm to humans, and fish is no exception.
She said that the worms found in fish were easy to spot, to the trained eye, and that a good sushi chef would see them and not serve those pieces.
At the end of the interview, she was asked if there was any kind of food she would always avoid. She said she wouldn't eat sushi from a source she didn't trust fully.
Me, I love sushi too much to ever turn down the chance to eat it :D
In the USA, almost all types of fish that are to be served raw are required to be frozen to kill some of the most common worm parasites.
The general idea that all fish have worms that are dangerous to humans isn't necessarily so, however.
Parasites are not universal to all forms of fish. Freshwater fish seem to have more issues than others (which is why salmon, which spends time in both fresh and saltwater, has these issues), and among saltwater fish, the kinds of parasites that show up in halibut, cod, grouper do not appear in some of species of other fish that are common for sushi and sashimi.
Wahoo, for example, has a stomach parasite that is almost universal, but we don't eat the stomach. If you were to use a fresh, raw, locally caught wahoo for sushi or sashimi, your risk of parasitic infection would be minimal. Same is true for most varieties of tuna, hamachi (Japanese Amberjack, which is not the same kind of amberjack caught in North Carolina, for example, that commonly has a worm infection but even that is not harmful to humans).
So, really, the safety of raw fish depends on how the fish is handled, stored, and the species of the fish. There is no generalization that covers all scenarios, but it can be very safe.
Eating raw fish, shrimp, lobster, & other can give you worms. A problem were I live. So all should be froze at 0f for 3 days or more or treated on the boat. But fresh is best. Firm soiled nice fresh smell to it. Best sliced & ate while the gills are still moving. But in the islands children 1st threw 6th are wormed at school. It is not a big problem but does happen. Not just from fish but ground raised pork. I think in America all fish sold are treated. Not fresh caught.
The key to sushi is the quality and the freshness. The fresher, the better.
Much the same rational as steak tartare. To quote Anthony Bourdain, "The key to a successful steak tartare is fresh beef, freshly hand-chopped at the very last minute and mixed table-side"
Note that sushi is saltwater fish. Eating raw freshwater fish is not a good idea.
Raw fish meat and chicken can cause intestinal worms wich create in your stomach in order to eat all undisposed fish meat or chicken that your body has not fully gotten rid of . They help by eating these leftover subjects but are pretty wierd to think about having and can start eating the body itself it's best to never eat raw fish chicken or meat of any kind and also to make sure it's fully cooked.