As with all food items, there is the potential for contamination. Sushi is generally a ready-to-eat item in that it does not undergo any cooking before service to the customer, and is safe to eat, provided that proper food safety procedures are followed.
There are two main issues with contamination that you should be aware of.
1. Cross-contamination: this occurs when either hands or equipment (and other food contact surfaces) are not properly clean and sanitized before use. This is generally the main reason that individuals may become sick from eating sushi.
2. Food borne pathogens: Whether they be viral, bacterial, or parasitic, raw seafood can be a vector for contamination's. This is why proper sanitary conditions must be met in order to maintain a safe food supply. For example, Anisakiasis, a microscopic larval worm that lives in marine creatures is one of the most nastiest and potentially fatal organisms associated with raw seafood. The United States requires, by law, that sushi grade items go through certain deep freezing techniques in order to destroy these micro-organisms. The only other way to destroy this organism is through cooking, which defeats the purpose of sushi.
Suffice to say, the regulatory requirements for sushi grade seafood are more strict than many other food items, and while it is still possible to become sick from these items, they are fairly rare.
A few recent (2010 & 2015) incidents found to be linked to raw tuna in sushi was suspected to be linked to the raw tuna being imported from Indonesia.