Infectious Disease

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Infectious Disease

Infectious diseases or infections occur when disease-causing microorganisms enter in the body and multiply. Microorganisms cannot be seen without a microscope. Most infections are transmitted through person-to-person contact, by consuming contaminated food & water and through insect or animal bite.

Microorganisms can be of many types – such as bacteria, viruses, fungi and protozoa.

Bacteria: are small single-celled organisms, which can only be seen with the aid of a microscope. Several types of bacteria are harmful – they can multiply in our bodies and produce toxic substance which can make us sick. However, most bacteria are harmless or even beneficial to us.

Viruses: are the smallest, containing the genetic material, and utilize our own cells to multiply

Source: US, Centers for Disease Control and Prevention

Fungi:  Multi-celled plant type of organism who cannot make their own food. They live in humans and animals to get their food. Some type of skin infections are caused by fungi.

Protozoa:  They are also unicellular organisms (one celled). Protozoa naturally live in water and some protozoal infections are transmitted through drinking contaminated water. Naegleria is a protozoa which causes devastating brain infection, usually from swimming pools or contaminated water enters in the nose.

Naturally, a huge number of microorganisms live inside or on the skin of our bodies.  Luckily, most of those microorganisms are harmless and many of them are actually beneficial to our health. These natural inhabitant microorganisms are collectively referred as normal flora.

There are many microorganisms that can cause disease (infections) to us. Those harmful microorganisms are in medical terms called pathogens.

Normal Flora

These organisms live in mutually beneficial relationship with our bodies.

Normal flora organisms are in many ways beneficial to our health. In general, normal flora organisms protect us from getting infections as they naturally occupy spaces on our skin, intestine, respiratory tract and other parts of our body which in their absence can be occupied by disease-causing microorganisms. Additionally, many normal flora organisms produce essential vitamins for us while living in our bodies.

Therefore, it is very important to keep the normal flora balance maintained. This natural balance of normal flora organisms can be disturbed as a result of many factors for e.g. misuse/ overuse of antibiotics – as antibiotics kills many of these normal flora organisms (see appropriate use of antibiotics) and individuals suffering with weak immune system disease (immunocompromised).

Source: US, Centers for Disease Control and Prevention

Cause of Infections

Infections are caused when disease-causing organisms enter in our bodies or attach on our skin.  Infection can be passed from person to person through air droplets (e.g. flu, common cold. Chicken pox), some can be transferred from insect bites (e.g. Malaria) or animal bites (Rabies). There are many which can be acquired from consuming contaminated food or water (e.g. Typhoid and Gastroenteritis).

Patients who have suppressed immunity for e.g. cancer patients and patients who are on immuno-suppressive drugs have higher risk of infections.

In humans, most infections are caused by bacteria and viruses.  Common infections, causes and mode of transmission are listed below:


Signs and symptoms of infectious disease differ depending on the causative organism.

However, most infections are characterized by fever and weakness.

Infections transmitted from person to person:

Most upper respiratory tract infections are transmitted from close person to person contact via air-droplets or as air-borne particles:

  • Flu (viral)
  • Common cold (viral)
  • Strep throat (bacterial)
  • Chicken pox (viral)
  • Tuberculosis (bacteria)
  • Pneumonia (viral and bacterial)

Many infections are caused by consuming contaminated food or water. Most digestive system related infections are caused by consuming contaminated food or water:

  • Typhoid fever (bacterial)
  • Cholera (viral)
  • Gastroenteritis (bacterial and viral)
  • Hepatitis (A and E) Jaundice (Viral)

Majority of skin infections are caused by exposed to dirty water, soil and unhygienic conditions.

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