What is the difference between a natural experiment and a quasi experiment??????????/

Natural occurs in a situation where participants can't be randomly assigned to the experimental conditions. The IV has been manipulated naturally, so the researcher takes advantage of the pre-existing independent variable. It's called natural because the variable would not have changed even if the experimenter was not interested. It's also the IV that's natural and it doesn't have to be the setting so participants could be tested in a lab.

An example is measuring white blood cell activity before people take exams (low stress condition) and then during exams (high stress) there's less white blood cell activity during exams so stress negatively affects the immune system.

Quasi is a type of experiment that compares people on an existing difference like age or gender and the effect on the DV is recorded. No one manipulates the variable, it simply exists. For example, comparing the anxiety levels between phobic and non--phobic patients, the IV of having a phobia would not have been brought about experimental manipulation.