Anyone of us can get TB . It doesn't matter what race or nationality you come from  or whether you're rich or poor and at any age. But for many reasons, some groups of people are at higher risk to get active TB disease. You are at  a high risk if : 

  You have close contact with people  who are infected with TB

   You have medical conditions that make your body less able to protect    itself from disease for example : diabetes, the dust disease silicosis or if you are undergoing treatment with  for example : diabetes, the dust disease silicosis or if you are undergoing treatment with drugs that can suppress the immune system , such as long-term use of corticosteroids.

You work in  or are a  resident  of long-term care facilties eg.clinics and certain hospitals.   
You  are underfed
You are an Alcoholic or an  IV drug user
You  live in poverty i.e.. You do not have access to Health Care (TB screening , TB treatment ).
You are malnutrioned or alcoholic which  may contribute to immunological vulnerability.
                                                HOW DO WE GET INFECTED ?
 
When an infectious TB patient coughs or spits, many small droplets of liquid are thrown into the air. These drops carry live TB bugs called as tubercle bacilli, which remain floating in air. f you are in close contact with the patient , then you are likely to breathe in these TB Ibugs and get infected.
 
In congested and poorly ventilated areas the bugs are more likely to remain in the air and increase in number as the patient continues to cough. Thus, everyone sharing the same room and breathing the same air runs the risk of breathing in tubercle bacilli.
 
                 WHAT HAPPENS WHEN WE GET EXPOSED TO THESE BUGS ?

The bacilli that enter the body are carried through the air passages to the air sacs
 (called "alveoli"), and get deposited there. Defence cells called as "macrophages", engulf these bacilli. An initial contest between the host (person  infected) and the parasite (TB bugs) begins, the outcome of which is crucial to decide if infection in that person will progress to disease or not.
The outcome depends on 2 factors:
 1.the infecting dose and
 2.fighting capacity of the macrophages against the virulence (strength) of the
    microorganisms

              WHAT HAPPENS WHEN THE BODY CANNOT DEFEAT THE BUGS?
 
If the capacity of the macrophages is low against the virulence of the bacilli, then even a single droplet (containing 1-3 bacilli) is adequate to cause infection. When the fighting capacity of  these macrophages is inadequate to destroy the initial few tubercle bacilli, they remain there and slowly multiply in large numbers. Ultimately, the macrophages burst, releasing the TB bacilli. Some of the macrophages, in turn, die in this process.

            WHAT HAPPENS NEXT TO THESE TB BACILLI?
 
 Some TB bacilli remain in the lungs, and some are carried in the drainage  "lymph node", lying besides the bronchi. In both the places (lung and lymph fluid of the  body called "lymph" to the nearest collection center called node), the presence of bacilli causes a reaction called an inflammatory process, wherein other defence cells of the body like monocytes begin to collect. These changes initiate the formation of what is called as a "granuloma".   These changes in the lung and in the lymph nodes are together known as the PRIMARY COMPLEX.This completes the process of infection.
 

           DO WE GET AN ACTIVE DISEASE ONCE WE ARE INFECTED?
   No, not all people. In most people, disease may not occur for a very long time.The TB
   bacilli can remain dormant in the primary complex for many years. In the event of
   lowering of the patients'defence system due to malnutrition or by another disease       
   such as HIV, these dormant bacilli then multiply and cause the active disease.