Herring is a type of fish. Many people eat different kinds of fish, but any consumption of meat should be considered. Animals including fish contain diseases just like humans do. However, people eat the fish and should consider the diseases that this fish may have. If consumed, the human could get sick especially if the meat is eaten raw or not fully cooked. One problem is gastritis. Gastritis is an infection on the lining of the stomach.
When this happens to someone, they would feel sick to their stomach and feel cramps. On the cruise ship, many people may get sick from one type of fish or meat being served. In this case, you could expect the relative risk would be 2.0.
Relative risk (RR) = Risk in exposed/Risk in unexposed
And, Risk = Number of cases (d)/ Total number of people at risk (N)
Therefore, RR for developing gastritis from herring consumption = Risk in Ate/ Risk in Did not eat
= (200/1000)/(100/1000)
= 2.0
NOTE: The risk can be calculated here because the total number of people who were at risk for falling ill is known. If the total number of people at risk of falling ill say in a situation where a certain food chain is contaminated then one cannot calculate risk but the odds of exposure is calculated instead (which should approximate the risk).