Here´s an interesting statement for you, no one has only ADHD! There are always at least two co-occurring conditions. This is something that everyone involved in either having ADHD, or living alongside the condition.
The biggest problem with co-occurring conditions is that doctors are often inclined to treat the co-occurring conditions first. This is incorrect. Even when the co-occurring condition is severe, you need to treat the severity, but then treat the underlying cause - the ADHD. The late Dr Stephen Copps who was the keynote speaker at a conference in South Africa in 2002 was emphatic that you should always treat the ADHD first, and then treat what´s left.
If you see something written by - or listen - to a medical professional - they will use the word comorbid to describe conditions that run alongside ADHD. One of the meanings of the word morbid is, ”relating to, or characteristic of disease” so it in that sense it is correct.
But, the common usage of the word morbid means grisly and gruesome. Now some might say I am being petty, but consider this. The two conditions that always walk along with ADHD are anxiety and depression. So to say that depression is comorbid is a double whammy! Better to use co-occurring which means the same and has no useless related meaning. Rant over.
Anxiety and Depression are always present with ADHD. In a few cases you might not qualify to wear an anxiety or depression badge, but you will be anxious all day, and you will feel depressed. There are other conditions that occur alongside ADHD. 
Other co-occurring conditions include behaviour disorders, such as Oppositional Defiance Disorder, and at the extreme there is Conduct Disorder. Pervasive Development Disorders are another group that occur alongside ADHD. This includes Aspergers syndrome, and two rare conditions, Childhood Disintegrative Disorder, and Rett Syndrome. Finally in this category there is PDD-NOS, which stands for Pervasive Development Disorder - Not Otherwise Specified - nuff said!
A rather distressing group of conditions are Tourettes Syndrome and Tics. The former is characterised by the foul use of swear words and inappropriate touching of the genital area. Tics occurs in two forms, there are vocal tics which are usually coughing and grunting sounds.
Motor tics can vary from a single muscle group twitching, such as eye blinking, to multiple muscle groups all subject to tics. Complex tics can involve hitting self and others. Although mentioned fairly down this list, Sensory Disorders are a significant co-occurring condition. This impacts what clothes you can wear, what foods you can and can't eat, whether you able to be affectionate or not, anything that is sensory in nature can impact your life. In some cases people who are tactile defensive can become tactile seeking as a way of combating the relationship and social effects of this difficult disorder.
This article is in a very important section of the website. Most people believe that ADHD is a solitary condition, like flu. If flu gets worse it develops into pnuemonia, which has many forms and some other conditions which make the pneumonia worse. ADHD is NEVER a solitary condition, it is not like flu, rather it is like pneumonia, it has other conditions that swirl around and tag along for the ride.
One big difference between ADHD and pneumonia is that your are born with ADHD and there is no cure, whereas you "catch" pneumonia, and you can get cured of it. Sometimes the co-occurring conditions mask the ADHD, leading to incorrect diagnoses and therefore wrong treatment.
I want to point out that there are currently more than 450 mental disorders - this is a massive amount! I have included an excerpt below from the Wikipedia mental illness page which sets out valuable definitions, and provides the current list for you to peruse:
The DSM is the American Psychiatric Association's standard reference for psychiatry, which includes over 450 different definitions of mental disorders.[1] The ICD published by the World Health Organization is the international standard system for classifying all medical diseases.[2] It also includes a section on mental and behavioral disorders.
The diagnostic criteria and information in the DSM and ICD are revised and updated with each new version. This list contains conditions which are currently recognized as mental disorders as defined by these two systems. There is disagreement in various fields of mental health care, including the field of psychiatry, over the definitions and criteria used to delineate mental disorders. Of concern to some professionals is whether certain mental disorders should be classified as "mental illnesses" or whether they would be better described as neurological disorders, or in other ways.
The excerpt above was found here: Wikipedia List of Mental Disorders
There is another factor that aggravates the issue of so many different conditions; the increasing number of specialists. ADHD occurs along a continuum, with many different types and flavours. This means that the specialist is looking for his or her condition that they specialize in, rather than looking at the big picture and determining what conditions are present.
In summary, there are four significant factors to take into account in this section, 450 mental conditions, a wide variety of specialists, and ADHD on a continuum, complicate not only getting the correct diagnosis, but getting the correct or appropriate treatment.