GCSE Science/Disease

What are infectious diseases?

Infectious diseases are diseases which can be passed from one person to another.

There are a number of organisms that can cause infectious diseases

• Fungi

• Bacteria
• Viruses

Fungi

A fungus is neither a plant or an animal, it is an organism in its own kingdom, the Fungi. They can be uni-cellular or multicellular. Like animal and plant cells, fungal cells are eukaryotic. This means that they have a nucleus that contains DNA. Fungi reproduce by producing spores. An organism usually becomes infected by a fungus via a cut or through being breathed into the lungs. The fungus feeds off of the cells of the infected organism.

An example of an infectious disease caused by a fungus is Thrush. This is caused by the yeast Candida. This normally survives in people's throats without any issue. If there are changes to the conditions in the throat, Candida can then start damaging the patient's throat cells, causing symptoms of the disease.

Bacteria

These are small, single-celled organisms. They are much smaller (approximately 10 times smaller) than animal and plant cells. Unlike plant and animal cells, bacteria lack a nucleus and membrane-bound organelles. This means that they are classed as being prokaryotes. Bacteria reproduce by splitting into two via a process called binary fission. This is very similar to mitosis, which is how cells in multicellular organisms replicate. Bacteria cause disease by growing and damaging the cells of an organism or by producing toxins.

There are many bacterial infections. One example is Strep Throat, which is a bad sore throat caused by the basterium Streptococcus.

Viruses

Viruses are many times smaller than bacteria. An electron microscope is needed to even be able see a virus. They do not have cells and cannot reproduce on their own, they rely on the host body cell reproducing them. They are made of a protein outer case protecting and enclosing a small amount of DNA or RNA (depending on the virus).

Viruses enter body cells and then effectively hijack the cell, causing the cell to reproduce vast quantities of virus. Eventually the virus will burst out of the cell, in the process killing the cell and harming the infected organism. This can cause symptoms of the disease. Alternatively the virus can affect the DNA of the organism, causing it to function abnormally.

An example of a disease caused by a virus is the common cold

Artificial defence against disease

Ignaz Semmelweis was a doctor who first came up with the idea of infectious diseases being spread easily. In the hospital he worked in, he was able to observe medical students performing dissections, then delivering babies. Many women died from diseases. Semmelweis believed these were passed on by the doctors unwashed hands. Upon telling the doctors to wash their hands, death rates fell.