Hello, readers. This is a continuation to where I stopped in my post titled HEALTH AND DISEASES: Introducing disease and how disease-causing factors influence each other. Today, I will be starting with the explanation on the difference between infection and disease.


Infection is not the same as having a disease. Before the disease develops, there is an incubation period in which the pathogen multiplies in the cells and tissues of the host. Think about what happens when you have been in a room with someone who is suffering from a cold. The infected person might sneeze and expel tiny droplets of mucus and saliva carrying the cold viruses on them.


You inhale these particles and the viruses get into your throat. There they attach to the epithelial cells in the mucous membrane. Like all cells in the body, these cells have cell surface membranes that contain many different protein molecules. The type of proteins present depends on the cell type, and on the genetics of the individual. The virus attaches only if right protein is present in the cell surface membrane.

The virus then enters the cell and begins a cycle of reproduction and colonization of new cells. But infection is not entirely one-sided. The body has a complex immune system. This might prevent the symptoms of the disease from ever developing. Even if some illness does follow, the immune system works to allow us to recover quickly.

The effects of disease

Infectious disease has two main effects. First, the pathogen can damage the cells of the body directly. The human immunodeficiency virus, HIV, gives rise to the symptoms of AIDS mainly as a result of destroying large numbers of a particular type of white cell. As a result, the immune system stops working effectively and this allows other infections or certain types of tumour to develop. Similarly, some of the symptoms of malaria result from a loss of red blood cells or the blocking of capillaries that supply particular organs in the body.

Alternatively, the pathogen can cause the release of substances that injure the body. Many bacteria, for example, produce toxins. Exotoxins are released by bacteria as they grow. Endotoxins, which form part of the bacterial cell wall, escape into the body when the bacterium dies and the cell wall breaks down.

Diphtheria is a good example of a bacterium that causes disease because it produces a toxin. It was a dangerous bacterial infection that used to kill many children until about 50 years ago. Since then, antibiotics and an effective vaccine have virtually eliminated it.

A diphtheria skin lesion on the leg. CDC - Public Health Image Library, Public Domain.

Diphtheria has an incubation period of 2 to 6 days, and then a sore throat and fever develop. A membrane forms across the throat and makes it difficult to breathe. As the bacteria multiply, they release a toxin into the blood. This toxin inhibits protein synthesis and results in damage to many parts of the body, particularly to the heart and the nerves. Interestingly, this toxin is produced only in bacteria of the species Corynebacterium diphtheriae that are themselves infected by a bacteriophage. The toxin is actually produced by one of the bacteriophage genes.


Tuberculosis (TB) is a disease caused by an infection with the bacterium Mycobacterium tuberculosis. It is transmited via airborne droplets or, more rarely, unpasturised milk. According to the World Health Organization, WHO, there are close to 10 million new cases of TB, and 1.6 million of them prove fatal.source In the UK and other more developed countries, the incidence of tuberculosis fell dramatically during the twentieth century. Many factors contributed to this decline. There was a general improvement in living standards during this time, and advances in medicine such as the discovery of effective antibiotics and vaccines have made tuberculosis treatable and preventable. However, there are now signs that the number of cases is rising again. What has gone wrong?

Part of the problem is that drug-resistant strains of mycobacteria have evolved. Surveys in selected sites in Asia show high levels of TB that is resistant to multiple drugs and so cannot be treated with the most powerful anti-TB drugs. The number of people being immunized has decreased and large numbers of people have become more susceptible because they have AIDS and the decreased immunity that goes with it. Experts predict a further increase in the number of TB cases because of higher rates of TB transmission to HIV positive people.

Signs and symptoms of TB

Tuberculosis can affect many of the organs in the body. In almost 80 per cent of cases it affects the lungs. Someone with primary tuberculosis has a fever, tends to lose weight and often has a persistent cough. They might cough up blood because the bacteria destroy the lung tissue.

The main symptoms of variants and stages of tuberculosis. Mikael Häggström., Public Domain

Treatment and control of TB

There is no completely effective vaccine against TB of the lungs. BCG vaccine (bacillus Calmette-Guérin) was developed in 1921. It is very use in preventing certain types of TB including the more severe types that can affect babies during the first year of life. The vaccine, which is a live but attenuated, or altered, strain of Mycobacterium tuberculosis, is given at or soon after birth to 85 per cent of babies worldwide. The vaccine provides cross-protection against leprosy, but is unlikely to protect adults against TB.

The disease itself can be treated successfully with antibiotics. Combination of drugs are used to discourage the development of strains that are resistant to particular antibiotics. Since tuberculosis is infectious, health workers trace all the people who have been in close contact with patients. These contacts can then be screened for the disease and treated if necessary.

In 1998, a group of researchers announced that they had decoded the entire genome of the tuberculosis bacterium. Knowing the sequence marks a new phase in the battle against one of the most successful pathogens that affects humans. The sequence contains 4 411 529 base pairs, which combine to form over 4000 individual genes. The genome is already yielding information that could prove invaluable to future research on anti-TB drug and vaccines.


It is difficult to decide whether viruses are living organisms at all. They do not show several of the characteristics that define a living organism: for example they don’t respire or feed. But they are more than just a collection of very complex chemicals. Their origins are also rather uncertain. Some biologist think they are stray bits of DNA that have escaped from the genomes of higher plants and animals.

Viruses vary considerably in size but they are all very small. The virus responsible for polio, for example, is one of the smallest and is only 20 to 30 nm in diameter. The herpes virus is much larger: its diameter is about 250 nm. In addition to their small size, viruses have a very simple structure. They consist of a piece of genetic material, DNA or RNA, surrounded by a protein coat (the capsid) and sometimes also by a membrane.

In order to multiply, a virus must take over the host cell. It uses the cell’s organelles and biochemical processes to make more virus particles. Viruses therefore have a great capacity to harm the cells and tissues of their host, and they cause a huge range of diseases, including influenza and AIDS.


AIDS, short for acquired immunodeficiency syndrome, is characterized by the destruction of vital cells in the immune system. It is caused by the human immunodeficiency virus (HIV). The disease was first recognized in the early 1980s and the virus itself was identified in 1986. It has spread dramatically. As of the end of 2010, an estimated 33 million people worldwide – 30 million adults and 3 million children younger than 15 years – were living with HIV/AIDS. About 70 per cent of these people (22.5 million) live in sub-Saharan Africa; another 15 per cent (4.9 million) live in South and Southeast Asia.

HIV is a retrovirus: it carries the enzyme reverse transcriptase. Once the virus infects a cell, often a T lymphocyte, this enzyme allows the cell to make virus DNA from virus RNA. The virus DNA is then inserted into the cell’s own DNA. Here it acts as a gene. It might do nothing for a long time but eventually it will cause the production of more virus RNA. This will result in thousands of new viruses that burst out through the cell surface membrane and infect other cells.

HIV is often transmitted from one person to another during sex, but the virus can also spread when drug users share dirty needles, when infected blood is transfused, or when contaminated blood is used to make blood products. When HIV enters the body. It can be many years – maybe 10 or 15 – before AIDS develops. A very small number of people are known to have been HIV-positive since the mid-1980s and have not so far developed the syndrome. Perhaps they never will, and AIDS researchers are trying to find out why. This might provide really important clues about how AIDS can be held in check in other people.

Signs and symptoms of AIDS

The table below shows the main stages in the development of AIDS from first infection with HIV.


Number of T helper cells per mm3 of blood 

Main features of stage

Aover 500

No symptoms but patient has antibodies to HIV in the blood.

B200 - 499Patient usually has fever or diarrhoea which lasts for a month or more. He or she might have infections such as thrush, caused by Candida, a type of fungus.
Cless than 200Likely to have one or more of the following diseases known to be related to AIDS: 1. Tuberculosis. 2. Karposi’s sarcoma, a form of cancer in which large black or brown tumours appear on the skin. 3. Pneumonia.


Treatment and control of AIDS

Drugs are available for the treatment of AIDS, and they are getting more effective all the time. However, they are expensive and can only slow the progress of the disease – there is currently no cure. An effective vaccine is thought to be a long way in the future because HIV can change its surface proteins and evade host immune responses. Gene therapy might provide some hope but, until it is developed, the current priority is to limit further spread by advising people how to reduce their risk of exposure to HIV.

HIV is capable of infecting another person only when it is passed inside lymphocytes or macrophages. Apart from blood itself, semen and vaginal secretions contain large numbers of these cell types, making body fluids high-risk sources of infection.

Practicing safe sex – using condoms during every sexual encounter – is very important. Using a barrier method of contraception not only prevents pregnancy, it cuts down the chance that HIV in one person’s semen or vaginal secretions will get into his or her partner’s blood. Drug users who share dirty needles are also at risk. Programmes that provide free, sterile needles on demand are designed to cut down HIV transmission by this route.

Main symptoms of AIDS. Mikael Häggström, CC0


Outbreaks of influenza or flu occur every year, with huge epidemics, or pandemics spreading throughout the world every 30 years or so. The disease is caused by a virus that affects the epithelial cells that line the nose and throat. After a short period of incubation, flu causes headache, fever and aching muscles and joints. In most years, a bit of rest and drinking plenty of fluids allows most people to get better within about a week. However, the old and the very young can be very ill and thousands of people die from flu every winter.

Flu passes between people rapidly because it is airborne. When we breathe out, sneeze or cough, tiny droplets of mucus and saliva fly into the air. If you have flu, these droplets contain flu viruses and this is how they are passed on to other people.

A global disease

The influenza virus has different forms. Two of these, Type A and Type B, are associated with major epidemics. Influenza epidemics are most often caused by Type A viruses. Type B is not as common but outbreaks of Type B influenza occur on a global scale. Such large-scale epidemics are known as pandemics.

One pandemic broke out in Europe in 1556 and lasted until 1560. Historians estimate that during this period almost 20 per cent of the population of the United Kingdom died of influenza. The worst flu pandemic that is recorded in recent history occurred in 1918-1919, just at the end of the First World War. Experts estimate that this very virulent form of flu killed between 40 and 60 million people worldwide. It affected just as many people who were in their 20s as the elderly and the young, spreading rapidly in the troubled conditions of the Western Front.

Symptoms of influenza with fever and cough the most common symptoms.Mikael Häggström, Public Domain

Protection against influenza

Why is it that you can have a single attack of measles and then be immune for life. But you can get influenza many times?

A person who has had influenza becomes immune to the strain of virus that caused it. However, the influenza virus has a very high rate of mutation. This means that different strains arise, each with slight differences in its outer protein coat. The body might be immune to the strain responsible for an outbreak of influenza one year, but this immunity does not protect against other strains that cause later outbreaks of flu.

Influenza vaccines are available. They are usually given to those at risk such as elderly people and those with asthma and heart conditions. The actual vaccine produced is effective only against the two or three strains of the virus common that year. It is necessary to have another injection with different vaccine the following year since new strains will probably have evolved. 

Thanks for reading.


Comments 2

I thought works towards AIDS vaccine were quite advanced. Do you have more information with this respect, by any chance?

15.09.2019 09:30

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