How much damage do 5 cigarettes a day cause?As a medical student, one of the most striking things that I've had to face is the incredible level of uncertainty surrounding a lot of our guidelines and practices in the field of medicine. There is a common adage that is told to first year students: "only about half of what you are going to learn in medical school is true -- the only problem is that we don't know which half." As such, the actual research literature surrounding most topics is full of contradicting studies and controversy.
One of the most prominent exceptions to this consistent uncertainty is the fact that smoking is absolutely horrible for your health. Surely, there is controversy with respect to the exact mechanisms through which smoking impacts us, and it is hard/impossible to predict the course of smoking-related illness in a given patient. (Especially in the development of cancer, there are a lot of random processes involved.) However, smoking has been demonstrated -- unequivocally and time and time again -- to shorten your life, damage your heart, destroy your lungs, worsen your looks, etc. etc. This finding is so consistent that including "smoking status" (whether or not you do) as a covariate is essentially a requirement in any observational study looking at outcomes for any kind of illness or treatment. As such, whether it is one cigarette a day or 50, the correct answer is always to decide not to smoke. You are literally paying money to destroy your life.
With that soapbox in place, the direct answer to your question really depends on the duration for which you've smoked. Most studies have shown that a big prognostic factor is the number of "pack years" in a smokers history. This is calculated by the number of cigarettes you smoke per day times the length of time during which you've been smoking. So, one pack per day for five years is about the same as five packs per day for one year. This number has been correlated with all the horrible problems listed above and then some. So, as you research all the terrible things cigarettes do, look at the average number of pack years included in the study and make whatever adjustments based on how long you (or the person you are asking about) has been smoking.
Now for the good news! A number of studies have shown that when you quit smoking, your body is able to slowly repair most of the damage. So, with time, your risks for all these horrible diseases will return back to levels as if you have never smoked at all. This nice gift of the universe is in direct contrast to a lot of other types of damage we can do to ourselves.