4.6.5 & 4.6.6 The courses of bacterial and viral infections


Tuberculosis (TB) is caused by the Mycobacterium tubercolusis bacterium.


  1. Mycobacterium tuberculosis is inhaled into the lungs in droplets of water & mucus from another person’s lung (droplet infection)


  1. TB begins to reproduce in the


  1. The bacteria produce toxins, which damage lung tissue & cause coughing, increasing the transmission of the


  1. The body launches an immune response to the TB


  1. Histamine release and inflammation occur (see 6.7)


  1. Macrophages enter the lungs in large


  1. The macrophages engulf the TB bacteria in large groups, forming a mass of tissue called a granuloma. The inside of the granuloma is starved of oxygen, which kills the bacteria.


  1. Once the bacteria are dead, the lung heals




  1. TB bacteria can survive inside macrophages as the cell wall of the bacterium is very thick and waxy and is resistant to the macrophage


  1. The bacterium can survive and reproduce inside the macrophage for many years without causing infection. When the immune system is weakened (by stress, malnutrition, or another disease – HIV is a common cause) the TB bacterium breaks out and re-infects the


  1. The bacteria reproduce too rapidly for the body to destroy


  1. The lungs are progressively damaged, which eventually leads to


  1. TB can also spread to the lymph nodes in the body, where it reproduces causing the disease scrofula

Course of infection for HIV

HIV is the Human immunodeficiency Virus, which eventually leads to Acquired Immunodeficiency Syndrome (AIDS).


HIV is spread by direct contact i.e. through sexual intercourse, blood-to blood transfer (tattoos, needle sharing, piercing & cut-to-cut transfer).

Once inside the bloodstream an HIV infection occurs in 3 distinct phases;


  1. The acute phase. HIV virus has a ligand (GP120), which attaches to a receptor (CD4) on the membrane of a type of white blood cell called a Helper T cell. HIV rapidly infects Helper T cells and the virus population increases quickly. At the same time the population of Helper T cells falls rapidly. The acute phase ends when the Killer T cells begin to recognise infected Helper T cells and kill them, which slows the replication of the


  1. The chronic phase. This can last for many years. The virus continues to replicate, but the Killer T cells keep the numbers in check. However, because the immune system is weakened other bacteria and viruses are more likely to infect the person (TB may reactivate at this point)


  1. The disease phase. As the numbers of virus increase and the numbers of Helper T cell fall the immune system becomes weaker and weaker. Eventually a second pathogen will infect the person (an opportunistic infection) which cannot be fought off. The person will die quickly from the secondary infection and this is the AIDS disease


The huge problem with HIV is that it mutates very quickly. Once inside the body the viral antigens change and the (already damaged ) immune system can’t keep pace with the changes. Another problem is that HIV attacks Helper T cells, which are crucial for activating the B cells and also play a role in activating Killer T cells. With low numbers of Helper T cell, the immune system cannot communicate effectively and this increases the ability of HIV to survive in the body.