Infectious Diseases
An infectious disease is any disease which is caused by a pathogen. The pathogen can spread within a population through direct contact, sneeze droplets or by drinking contaminated water, for example. Examples of infectious diseases include the viral disease HIV/AIDS and the bacterial infection tuberculosis.
Pathogens
A pathogen is any organism which causes disease. Infectious diseases are always caused by pathogens, which can spread from person to person to infect others. Non-infectious diseases, such as lung cancer or diabetes, are not caused by pathogens and tend to be more influenced by genetics or lifestyle factors.
COVID-19 is caused by the coronavirus pathogen and is spread via lipid droplets.
Pathogens include bacteria, viruses, protoctists and fungi. Here are some examples of pathogenic organisms from each of these kingdoms:
Bacteria - Mycobacterium tuberculosis causes TB
Virus - HIV causes AIDS, COVID-19 causes coronavirus, the influenza virus causes seasonal flu
Protoctist - Plasmodium causes malaria
Fungi - Trichophyton causes Athlete’s foot
HIV and AIDS
Access to clean needles is a way of preventing the transmission of HIV among drug users. Image credit: Ricky Carioti/The Washington Post)
Human immunodeficiency virus (HIV) is a deadly virus which can weaken a person’s immune system by destroying a type of immune cell called T helper cells. It is spread through the transmission of infected bodily fluids (e.g. during sexual intercourse, sharing of needles or blood transfusion). HIV doesn’t kill the patient directly, but weakens the immune system to an extent that the patient is unable to fight off other infections which normally would not pose a threat - these are known as opportunistic infections.
When the patient’s immune system has weakened to a particularly low level (this is seen by a low T helper cell count in hospital blood tests), the patient has developed a disease called acquired immunodeficiency syndrome (AIDS). The time between the initial infection and the onset of AIDS symptoms (the latency period) varies greatly between individuals but is usually around 10 years. The length of the latency period depends on the individual’s age, strength of their immune system and access to healthcare.
The symptoms of AIDS follow a series of stages:
The person will first develop less serious, minor infections which will gradually become more and more severe.
As the patient’s body tries to fight off more infections, the lower their T cell levels drop.
Eventually their immune system will be so weak that they will die of an opportunistic infection because they do not have enough immune cells to defend themselves.
HIV Replication
The HIV virus consists of a core of RNA and enzymes (reverse transcriptase and integrase) which is enclosed in a protein coat called a capsid. Surrounding the capsid is an outer layer called the envelope which contains attachment proteins - these proteins are crucial for the virus to enter host cells (they act like little keys to access our cells).
If a person with HIV exchanges bodily fluid with another person, HIV can infect the second individual and will be present in their bloodstream. The HIV virus uses its attachment proteins to enter human immune cells (specifically the T helper cell) by binding to receptors on the T cell. The capsid is released into the cell, where it breaks apart to release the RNA and enzymes. The enzyme reverse transcriptase converts the RNA into DNA. The single-stranded DNA is converted into double-stranded DNA which the enzyme integrase can insert into the DNA of the T cell. The T cell now has the ‘instructions’ (genes) to produce viral proteins. The viral DNA is transcribed and translated and the viral proteins are used to build new virus particles, which move out of the T cell and infect other cells.
Tuberculosis (TB)
TB spreads when an uninfected person inhales lipid droplets from an infected person when they sneeze or cough.
TB is caused by a bacterium called Mycobacterium tuberculosis. It is spread through lipid droplets i.e. when an infected person sneezes or coughs. The lipid droplets are inhaled by another person, causing the bacteria to move into the lungs. Once they are in the lungs, the bacteria are engulfed by a type of white blood cell called a phagocyte. Phagocytes usually work by digesting and killing the pathogen, but Mycobacterium tuberculosis is able to disrupt this process. The bacteria are able to survive and replicate from inside the phagocytes.
The time between infection with M. tuberculosis and the onset symptoms can vary between individuals - there is a latency period between a few weeks to many years. Initial symptoms include a cough and fever which is caused by inflammation of the lungs. The lungs become progressively more damaged, leading to respiratory failure and sometimes death. It can also spread to other parts of the body, such as the kidneys and the brain.
Often the symptoms of TB do not appear immediately. Our body is able to seal off infected phagocytes inside structures within our lungs called tubercles. The bacteria within the tubercles lie dormant for a period lasting up to a few years. The bacteria then become reactivated - reactivation can be stimulated by a weakened immune system (e.g. due to AIDS).
Did you know…
Mary Mallon, a US Irish-immigrant who earned the nickname ‘typhoid Mary’ was an asymptomatic carrier of typhoid fever. She spread the disease by working as a cook in New York. Every family she worked for would become ill and many died, before Mary slipped away to evade the health authorities and work for another family. She was eventually caught and spent a third of her life in quarantine facilities. Image credit: Getty Images
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