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Viral diseases

Viruses are not considered to be living organisms because they do not complete all of the seven life processes: movement, respiration, sensitivity, nutrition, excretion, reproduction and growth. This means they can only reproduce by invading living cells.

We say 'strains' of virus and not species. A strain is made up of a genetic material core, DNA, which is surrounded by a protein coat.

Viral infections cannot be treated by antibiotics.

Influenza (flu)

Influenza viruses are spread in droplets of body fluids from coughs and sneezes. The droplets are inhaled or transferred by hand to the nose, mouth or eye. Flu can be a very serious illness and so those who are vulnerable, for instance the elderly, are offered flu vaccinations. There is a new vaccination every year because the virus is continuously mutating into new strains.

Tobacco mosaic virus

The tobacco mosaic virus infects tobacco and other closely related plant species, such as tomatoes and peppers. It is transmitted by contact between plants, either naturally or through the hands of farmers. It infects the chloroplasts of plant leaves and changes their colour from green to yellow or white in a mosaic pattern. It can also make leaves crinkle or curl up.

This reduces the plant's ability to photosynthesise and grow properly, which reduces the crop yield for farmers. There is no cure, so farmers must try to reduce the infection to their crops or attempt to reduce the spread of the virus.

Tobacco mosaic virus is used by scientists as a model organism in order to help them understand the spread of diseases.

HIV/AIDS

HIV stands for Human Immunodeficiency Virus. It is an example of a sexually transmitted infection (STI). This infection is transmitted by body fluids, often during unprotected sex, but also through cuts and injecting drugs using shared needles. Immediately after infection, people often suffer mild flu-like symptoms. These pass and people might not know they are infected for some time.

AIDS stands for Acquired Immune Deficiency Syndrome. Months or years after infection, the HIV virus becomes active and starts to attack the patient's immune system. HIV at this point has become AIDS.

There is no cure for HIV /AIDS although many scientists are trying to find one. Currently, infected people are given antiviral drugs, which can slow the development of AIDS.

Bacterial diseases

Not all bacteria are pathogens, which cause disease. Many bacteria, like those found in the intestines, are useful. All bacteria are prokaryotes so do not have a nucleus. Unlike viruses, they are cells but they are too small to be seen without a microscope.

Population growth of bacteria

In the correct conditions - with warmth, moisture, nutrients - bacteria can multiply rapidly. The human body can provide these conditions, eg in a cut. Each bacterium may divide every 20 minutes in ideal conditions.

So, after one hour a single bacterium could have reproduced to give eight bacteria.

Bacteria will only divide this rapidly when conditions for growth are perfect. Eventually nutrients and space start to run out and waste products from the bacteria poison and kill them.

Salmonella

Salmonella is a genus of bacteria that cause food poisoning. Symptoms of food poisoning include abdominal cramps, vomiting and diarrhoea. It is often found in unhygienic kitchens, undercooked foods such as meat, eggs and poultry, or the same foods that have not been reheated properly. To prevent the spread of salmonella in the UK, all poultry are vaccinated against it. Cooking food thoroughly, after preparing it in hygienic conditions, is the best way to avoid illness.

Crown-gall disease

Agrobacterium is a genus of bacteria that causes tumours in plants. Agrobacterium is a Gram-negative type bacterium. The tumours it causes are called galls and are thought to be produced when a plasmid from the bacterium transfers DNA into the host cells. The bacterial DNA that is transferred into the host genome carries the genes to make the gall. The tumour often grows at the junction between the roots and stem (the crown) of the plant. Many varieties of plants are affected, including grape vines.

Protist diseases

Protists are a group of microorganisms that have features that belong to animals, plants and fungi. Some are like animals, others more like plants and some, called moulds, are closest to fungi. They are all eukaryotic, which means they have a nucleus.

Malaria

Malaria is spread by mosquitoes which carry the plasmodium protist. These are often found in areas with higher temperatures like Africa, Asia, and South and Central America, but not the UK. Mosquitoes suck blood containing the protists from an infected person. They pass the protist to other people they suck blood from. The mosquitoes do not become ill and are called vectors because they transmit the disease.

The symptoms of malaria include a fever, sweats and chills, headaches, vomiting and diarrhoea. Damage is also caused to the blood and liver. After decades of research scientists developed the first malaria vaccine in 2021. Infection continues to be reduced by stopping individuals from being bitten. People sleep under mosquito nets and wear insect repellent to avoid bites. Antimalarial drugs are also taken, which treat the symptoms and can prevent infection.

Fungal diseases

Not all fungi cause disease. Yeast is a single-celled fungus that is economically important because we use it to make some bread and beer. Not all fungi are single-celled. Some, such as mushrooms, are multicellular and are much larger. Their cells are still tiny, like yeast cells, but there are many more of them. Fungal cells have a nucleus and so are eukaryotic.

Athlete's foot

Ash dieback

The spread of ash dieback was covered on in March 2016.