A-level Biology/Human Health and Disease/infectious diseases

An infectious disease is a disease resulting from the presence of pathogens including viruses, bacteria, fungi, protozoa and multicellular parasites. These pathogens are able to cause disease in animals and/or plants.

Infectious pathologies are usually contagious diseases due to their potentiality of transmission from one person or specie to another. Diseases can be transmitted in a variety of ways, from drinking contaminated water to sexual contact.

The four diseases you are required to know are relevant because they are the ones of current concern - they are all in epidemic or pandemic status. Due to international travel, diseases can be spread round the entire world very quickly, as in the SARS incident of 2002/2003. Some bacteria are growing resistant to the use of antibiotics, which used to be an effective way of stopping disease spreading.

Cholera

As Cholera is a water-borne disease, it occurs where people do not have access to proper sanitation, a clean water supply or uncontaminated food. The bacteria pass through the stomach (if the contents are sufficiently acidic (less than PH4.5) the bacteria is unlikely to survive) and reach the small intestine. Here they multiply and release a toxin know as choleragen, which disrupts the epithelium functions so that salts and water leave the blood.

This causes severe diarrhoea which leads to dehydration, and can be fatal within 24 hours. Fortunately, treatment for cholera is relatively simple; the disease can be controlled by giving a solution of salts and glucose intravenously to rehydrate the body (Oral Rehydration Therapy). There is a vaccine available for some strains of cholera, but it only provides short-term protection.

Strains

There are more than 60 different strains of the pathogen that causes cholera, and there have been 8 pandemics of cholera, mostly caused by untreated sewage water.

Features

Table 1: Features of Cholera

Pathogen	Vibro cholerae
Transmission method	food and water borne
Incubation	1–5 days
Symptoms	Severe diarrhoea, loss of water and salts, dehydration.
Annual incidence/mortality worldwide	5.5 million/ 120,000

Tuberculosis

Tuberculosis is an incredibly invasive disease - it starts with a primary infection in the lungs and quickly spreads to the lymph nodes, bones and gut. It often strikes HIV-positive people when their immune system begins to weaken and in areas where there is no proper food availability.

It spreads via aerosol droplets and unpasteurised milk and is particularly prevalent in overcrowded areas. People suffering malnutrition are more susceptible. Infections have fallen now because of vaccine introduction in the 1950s. However it is estimated that 30% of the world's population is infected with TB without showing any symptoms of the infection; people with this inactive infection do not spread the disease to others.

Resistance

Tuberculosis is unfortunately showing a comeback, and this is thought to be due to a variety of factors.

These include;

• Breakdown in the tuberculosis vaccination and control program
• Poor housing causing overcrowding
• The AIDs epidemic weakening immune systems and allowing it to be more prevalent
• Some strains are now resistant to antibiotics

The last bullet point, about resistance, is particularly important. When antibiotics attack bacteria, any that are resistant to them survive, and thus multiply and possibly create a new strain of tuberculosis that is antibiotic resistant.

Features

Table 2: Features of Tuberculosis

Pathogen	Mycobacterium tuberculosis, Mycobacterium bovis
Transmission method	airborne droplets
Incubation	2–6 weeks
Symptoms	Coughing up blood, shortness of breath, fever, chest pain and sweating.
Annual incidence/mortality worldwide	8 million/2 million

Malaria

Infection stages

Take a look at the diagram on the right, showing the infection cycle of malaria. Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infected female Anopheles mosquitoes.

There are four parasite species that cause malaria in humans:

1. Plasmodium falciparum

2. Plasmodium vivax

3. Plasmodium malariae

4. Plasmodium ovale.

Plasmodium falciparum and Plasmodium vivax are the most common. Plasmodium falciparum is the most deadly.

When a female mosquito stings a human (to take a blood meal), either of the following event may occur:

1. In case of an infected person, the mosquito obtains both male and female gametes of Plasmodium along the blood it sucks up. Then, the gametes fuse in the mosquito’s stomach, forming thousands of immature malarial parasites which invade the mosquito’s salivary glands.

2. The other event that could occur is that an already infected mosquito with immature malarial parasites, injects them into a healthy person. The immature malarial parasites then undergo maturation in the person's liver.

Obviously, the 2nd event then returns to the 1st event and eventually results in a transmission cycle.

Features

Table 3: Features of Malaria

Pathogen	Plasmodium
Transmission method	Insect vector
Incubation	1 week - 1 year
Symptoms	Fever, nausea, headaches, sweating, spleen enlargement, muscle pain
Annual incidence/mortality worldwide	300 million/1.5-3 million

Control

There are 3 main ways of controlling the vector, and thus of controlling malaria;

• Avoid being bitten by mosquitoes, either by using insect repellent or mosquito nets.
• Reduce the number of mosquitoes, by destroying their breeding grounds, chemically (using a chemical that kills mosquito larvae) or physically (draining the water areas).
• Use drugs to prevent Plasmodium affecting people
• Draining swamps which serve as breeding sites for mosquitoes.
• Spreading oil on the ponds or swamps which prevents the larvae or pupa to breath, and eventually die.

Eradication program

Preventative drugs have been partially successful in breaking the transmission cycle - these drugs inhibit the parasite spreading in the body, some by inhibiting protein synthesis and some by stopping the sexual reproduction inside the mosquito.

Failures

• Many of the drugs used to treat it are now not working because malaria has built up a resistance
• Mosquitoes also became resistant to the insecticides used to treat it.
• The program was unpopular because due care towards indigenous people was not taken (at one point, teams spraying DDT were killed by villagers angry with them).
• When the disease was temporarily eradicated, people who had immunity lost it and when the disease returned (as the program was not successful) they suffered and some died as a result

Acquired Immune Deficiency Syndrome

Acquired Immune Deficiency Syndrome (AIDS), is a syndrome caused by the retrovirus Human Immunodeficiency Virus (HIV).

HIV

HIV pathogens infect and destroy the T helper cells of the immune system, and without these the immune system does not respond adequately to infection.

When T-Cell numbers are low, the body is particularly vulnerable to infection by anything from the common cold to tuberculosis. Thus, AIDS is not a disease, HIV is the virus that causes AIDS which is a syndrome. Affects the CD4 receptor cells.

Features

Table 4: Features of HIV/AIDS

Pathogen	Human immunodefieciency virus (HIV)
Transmission method	Exchange of body fluids (sexual intercourse, intravenous needle sharing, blood transfusions)
Incubation	HIV has a few weeks, but AIDs may not develop for up to ten years
Symptoms	HIV - fever and then none AIDs - hugely increased susceptibility to disease, such as pneumonia and TB.
Annual infected/new incidence/mortality worldwide	33.4 million/6 million/2.5 million

Treatment

HIV and subsequently AIDS cannot be cured, but the spread of AIDs can be slowed down with a variety of drugs, providing an increased life expectancy. This results in a virus that is difficult control, and is thus best prevented. People can be educated to use condoms and other means of reducing the risk of infection during intercourse.

Statistics

• 5.9 million children are estimated to have been orphaned by AIDs
• 25% of Zimbabwe is infected with HIV
• 34 million people are infected with HIV in sub-Saharan Africa.

Transmission and testing

Transmission is spread by intimate human contact and HIV cannot survive outside the body - transmission is only possible by direct bodily fluid exchange, most commonly during sexual intercourse, across the placenta and intravenous needle sharing. Testing is done via a blood test, but this only becomes available several days after the initial infection. This testing is offered to those who think they might have HIV, and they are often asked to contact sexual partners and inform them that they should get tested, as HIV caught early can be slowed down.

Pregnant women

HIV positive women in countries like the UK are advised to not breastfeed their children, since HIV can be transmitted this way as well since viral particles have been found in breast milk. However, third world countries have many more risks that breast-feeding negates that the risk of HIV is an acceptable one.

HIV positive women should take antiretroviral drugs (nevirapine) before delivery

Antibiotics

Antibiotics are selective toxins, killing or disabling the pathogen without harming the host. Only a few work on viral infections, and are derived from living organisms. Antibiotics are substances produced by the microbes to kill or retard the growth of other harmful microbes.#DEF:An antibiotic is a therapeutic agent,made by organism,that kill and inhibit the growth of microorganisms

How they work

Antibiotics work against bacteria and viruses in the following ways;

• Synthesis of bacterial walls
• Plasma membrane function
• Protein synthesis
• Enzyme function

Resistance

Pathogens can develop resistance to antibiotics, by developing enzymes for destroying penicillin for example. An example of this is MRSA, which is a bacteria that is resistant to four of the most popular antibiotics due to their inappropriate use and people not finishing their course of antibiotics – leaving antibiotic resistant bacteria to grow and spread.