Infections and Infectious Organisms

We have selected the following expert medical opinion based on its clarity, reliability and accuracy. Credits: Sourced from the MedSquirrel staff. Please refer to your own medical practitioner for a final perspective, assessment or evaluation.

What is an infection?

An infection occurs when a foreign organism (such as a bacteria or virus) colonises, gets nourishment from and reproduces inside the body. Infection does not necessarily lead to disease, but when the organism damages cells and tissue, it does.

Among the millions of types of organisms, only a few are able to infect our bodies and even fewer are able to cause disease. Those which do are called pathogens. Pathogens usually enter the body through openings to the environment such as the nose, mouth, eyes and wounds. They then migrate to tissues which they can more easily live in.

Infections can be divided into primary and secondary infections. Primary infections are caused by the first organism to invade the body. Secondary infections are caused by organisms that colonise the body due to a weakened immune response or other complications caused by a primary infection. For example, flu, a primary infection, can cause infection of the lungs or airways by bacteria, leading to bronchitis or pneumonia.

What is a virus?

A virus is the smallest and most primitive type of infectious organism. There are thought to be millions of types of viruses, found almost everywhere on earth. These can infect anything from the smallest bacteria to mammals, reptiles and plants.

Viruses cannot reproduce and survive on their own in the way a bacteria, plant or animal can. Instead, they consist of only a few genes packaged tightly into an outer coating made of protein. The coat serves to protect the viral DNA and help the virus to break into and enter a host cell. Once inside, their genes take over the replication machinery of the cell, causing it to produce more viruses. In the process, the cell dies, either because it cannot sustain both itself and the virus, or because the virus causes it to swell until it bursts. New viruses are then released, which spread into the environment and infect other cells.

What is a bacteria?

Bacteria are microscopic, single-celled organisms which are able to replicate and survive on their own. After viruses, they can be considered the next simplest infective agent. Bacteria can be found in almost every environment on earth, from the soil to the bottom of the ocean, on and in plants and animals, and even in radioactive waste and the atmosphere.

Most bacteria are essential to life on earth. They are extremely important to cycling nutrients, from causing the decomposition of dead animals and plants, to creating new nutrients by turning nitrogen in the air into fertilizer that plants can use. It is thought that bacteria where the first organisms to begin producing oxygen millions of years ago, thereby creating a world that was hospitable to other, more advanced life.

Surprisingly, the majority of bacteria living in your gut and other parts of your body are actually good for you. They keep other dangerous bacteria at bay by outcompeting them, or even producing their own antibiotics to kill their competition. These bacteria also help regulate your immune system, improve your overall health and produce nutrients which you may not usually be able to acquire from your diet. This is one reason why bacteria are used to create foods such as yogurt and other fermented foods.

Less than 1% of bacteria can actually be considered dangerous. The few types of bacteria that can make you sick cause diseases such as pneumonia, cholera, tuberculosis and food poisoning, amongst others. They cause disease by invading tissue and releasing toxins which damage cells and organs.

Are there other infectious organisms?

Yes, there are a few. Fungi, protozoa and parasitic worms are more complex than viruses and bacteria. Infection by these organisms is not as common, however, and is often linked to unhygienic environments, contaminated foods and insects.

Fungal infections most often affect the skin and nails and sometimes the airways. They do not usually cause severe disease, but in some cases can, especially when infecting the lungs. Fungi are responsible for common conditions such as athlete’s foot and ringworm.

Protozoa are usually contracted from contaminated water or bites from insects like mosquitos. These organisms cause diseases like malaria and amoebic dysentery.

Parasitic worms are internal parasites and can be contracted from soil, water, raw meat and infected surfaces. Proper sanitation, cooking food thoroughly and avoiding swimming in stagnant water can efficiently prevent them.

What do antibiotics treat?

Antibiotics specifically treat bacterial infections and have no effect on viruses. Taking an antibiotic for the flu is therefore unnecessary and will have no effect, unless your doctor has reason to believe that a secondary bacterial infection is developing. This can be difficult for a doctor to determine, and so they may prefer to be cautious and prescribe antibiotics just in case.

Over prescription and incorrect use of antibiotics can give rise to bacteria that are resistant to these drugs. It is therefore extremely important to take antibiotics exactly as they are prescribed.

Unlike antibiotics, which can kill a broad spectrum of bacteria, antivirals, prescribed for certain viral infections, are often very specific to the virus they target. This is why antiretroviral medication can not be used for the flu, for example. Because of this specificity, many viral infections cannot be directly treated.

Why do we get sick?

We get sick due to an infection by a foreign, disease causing organism. The most common symptoms of sickness, such as a runny nose, cough, fever and fatigue are, however, not caused directly by the infecting organism, but are rather part of your body’s immune response. For example, a runny nose results from your body trying to flush an infection out with increased mucous production.

Microorganisms can cause disease in many ways, including by releasing toxic substances, invading cells and tissues as well as releasing signalling molecules that can affect the whole body. These are the effects that damage the body, and are why it is essential to ensure the infection lasts for as short a time as possible, and specific treatment for the infection, rather than its symptoms, receive focus.

Why are we more likely to get sick in winter?

Scientists are still unsure why we tend to catch colds and flu mainly in the winter. There are, however, a number of hypotheses, and as with most things in biology, the answer is more likely to be a complex interaction of all of them, rather than any specific one in particular.

It is thought that cold, dry air protects viruses and bacteria, allowing them to survive in the environment longer. The same conditions may also dry out the organisms, making them lighter and able to travel further. Cold, dry air also prevents mucous secreted in the nose, etc., from being as effective at clearing out inhaled material, giving the organisms more time to gain a foot hold. Another reason is that people spend more time indoors in winter, creating a perfect environment for pathogens to spread from one person to the next.

When should I go to the doctor for a cold or the flu?

Colds and flu will usually clear up on their own but, in some cases, they may become more severe or result in secondary infections. If you are unsure whether to go to the doctor or not, it is always better to be cautious – rather pay a visit to your doctor than wait until an illness becomes really problematic. This is especially important for individuals who are at greater risk of developing serious complications, including young children, the elderly, immunocompromised individuals, pregnant women and persons suffering from respiratory conditions, such as asthma.

If you are an otherwise healthy individual, and experience any of the following symptoms, you should contact your healthcare practitioner:

• You are not recovering, your symptoms are not subsiding, or are getting worse
• Unrelenting fever: If the fever does not subside after a few days or respond to over the counter medication
• Persistent vomiting or diarrhoea
• Shortness of breath
• Worsening or persistent cough

See your doctor immediately or visit the emergency room of you experience any of the following:

• Chest pain
• Severe weakness
• Difficulty breathing or breathlessness
• Severe headache
• Severe rash

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