Firstly, it is important to acknowledge that the active molecule in alcohol is called ethanol. It is this molecule that procures the effects observed when a person is drinking alcoholic beverages. As an example, the mention 5% alc./vol. found on a bottle of beer means that this beer contains 5% of ethanol and 95% of other ingredients, mainly water.
Ethanol at a certain percentage is highly efficient to kill a wide range of microorganisms. Experiments have showed that 70% vol./vol. ethanol, meaning a mixture of 7 volume parts of ethanol with 3 volume parts of water, is the best concentration to successfully kill microorganisms, including parasites. Consequently, in microbiology laboratories and hospitals, successful disinfection is made using 70% vol./vol. ethanol. In order to effectively kill microorganisms, they have to be in direct contact with 70% vol./vol. ethanol. The mechanisms involved in this type of disinfection include the denaturation of the microorganism’s proteins, ultimately leading to its death. However, alcohol is usually not able to destroy all type of bacterial spores or parasitic cysts. Furthermore, the killing activity of ethanol greatly decreased below a 50% concentration.
Searching over the Internet could give the impression that alcohol consumption might help getting rid of parasitic diseases as many people testify that parasites were coming out of their body after getting drunk. In fact, as massive alcohol consumption cause usually gastrointestinal symptoms, many worms can get out of the body at that moment leading to the wrong impression that parasitic illness is cured. However, many worms and eggs usually remain within the body, even though some of them might get out. Consider this, it is logically hard to believe that we can obtain a concentration of ethanol higher than 50% directly in contact with parasites hidden in many cells and organs within the human body without killing it. Furthermore, remember that ethanol is inefficient on many parasitic cysts.
It is interesting to mention that one of the basis of this thought that alcohol can get rid of parasites comes from a recent well mediatized scientific study where it has been found that ethanol is used successfully by fruit flies as a self-medication in order to killed the parasitic wasp larvae living inside them. However, fruit flies are resistant to higher dose of alcohol by body weight than human. There are also no scientific grounds for any mechanism of this kind in humans.
Finally, the different effects of alcohol consumption on the human body are now quite well understood. Interestingly, it has been showed that drinking too often or too much alcohol contributes to weaken the immune system. As such, it has been established that chronic alcoholics and heavy drinkers are more prone to acquire infectious diseases as their immune system are less able to cope with infection. It has also been established that the immune system is affected for at least 24 hours after having drink too much, even though it is a very punctual event. However, as the immune system is a key in the fight against parasites, drinking alcohol can do more bad than good in that case.
Sources: Centers for Disease Control and Prevention
Milan, N. F., Kacsoh, B. Z., & Schlenke, T. A. (2012). Alcohol consumption as self-medication against blood-borne parasites in the fruit fly. Current biology, 22(6), 488-493.