Regarding AIDS, it is generally difficult to contract, unless through transfusion. If blood containing the HIV virus is transfused, then the risk in one occurrence is about 90%-100%. Other methods, such as vaginal intercourse, anal intercourse, needle stick injuries, and mother-to-child transmission, have a relatively low probability of infection per occurrence. The risk of typical vaginal intercourse is below 0.1% per occurrence, whereas the risk of anal intercourse is slightly higher, around 1%-3%. AIDS is mainly transmitted sexually.