Theory:

The various targets of the microorganisms:
The microorganisms which are the causative agents of disease enter the body by the various means as studied previously. After entry, these organisms go to various possible places, such as tissues and organs. The different species of microbes seem to evolve to home on different parts of the body. This selection depends on the point of their entry.
 
The microbes that enter through the nose from the air mostly go to the lungs. This is seen in the bacteria causing tuberculosis. A few bacteria like typhoid causing bacteria enter through the mouth, and stay in the gut lining.
 
Certain viruses go to the liver and cause jaundice. But it is always not the same case. Infections like HIV comes into the body via the sexual organs or infected blood or syringes spread to the lymph nodes all over the body.
 
The protozoan that causes malaria enters the body via the mosquito bite and affects the liver and red blood cells. The virus which causes Japanese encephalitis, or brain fever, also invades the body through a mosquito bite and infects the brain.
Signs and symptoms based on the targets:
  • The various signs and symptoms of the disease depend on the tissue or organ which the microbe targets.
  • The symptoms will include cough and breathlessness if the lungs are the targets.
  • If the liver is targeted, it leads to jaundice.
  • If the brain is targeted, it causes headaches, vomiting, fits or unconsciousness.
Thus based on the symptoms and signs of an infection we can know which tissue or organ is affected and what functions are carried out by this tissue or organ.
Common effects of the body:
Besides these tissue-specific effects of infectious diseases there are some common effects which depend on the fact that the immune system of the body is activated in response to infection. When the immune system is activated, it recruits many cells to the affected tissue so that these cells kill the disease-causing microbes. This process of recruitment is known as inflammation.
During this process, there are certain local effects such as swelling and pain, and certain general effects like fever.
 
During certain cases, the tissue-specificity of the infection leads to very general-seeming effects. For example, during the HIV infection, the virus attacks the immune system and damages its functioning. Thus, most of the effects of HIV-AIDS are because of the inability of the body to fight off the minor infections which we face. Each and every small cold can become pneumonia. A minor gut infection can lead to diarrhoea with blood loss. Thus, all these infections ultimately kill the person who is suffering from HIV-AIDS.
 
It is also important to remember that the severity of the manifestations of diseases also depends on the number of microbes present in the body. The manifestations will be minor or unnoticed when the number of microbes in the body is very less. If the number is high, the disease can be severe enough to be life-threatening. Thus, the immune system is a major factor that determines the number of microbes surviving in the body.