Disease: AIDS (Acquired ImmunoDefeciency Syndrome) 

Pathogen: Human Immuno deficiency virus (HIV). 

Reason: 

Due to decrease in the number of helper T lymphocytes, the person starts suffering from infections that could have been otherwise overcome such as those due to bacteria especially Mycobacterium, viruses, fungi and even parasites like Toxoplasma.

The path of this pathogen and its spread andeffect on the human body:

• After getting into the body of the person, the virus enters into macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase.
• This viral DNA gets incorporated into host cell’s DNA and directs the infected cells to produce virus particles.
• The macrophages continue to produce virus and in this way acts like a HIV factory.
• Simultaneously, HIV enters into helper T-lymphocytes (TH), replicates and produce progeny viruses.
• The progeny viruses released in the blood attack other helper T-lymphocytes. This is repeated leading to a progressive decrease in the number of helper T lymphocytes in the body of the infected person.
• During this period, the person suffers from bouts of fever, diarrhoea and weight loss.