How do infectious diseases spread?

Infectious diseases are caused by microorganisms that invade the body and develop disease. Following the incubation of the illness in patient zero, an infectious disease can be transmitted stealthily in a number of ways.

We are all familiar with how the COVID-19 virus can be spread: droplets that are coughed or sneezed out by individuals who have the disease, or through contact with contaminated surfaces. Other infectious diseases, like malaria, are transmitted through other vectors, such as mosquitoes. 

With the new COVID-19 variants, we are now seeing increased transmissibility. So we’ve put together a handy guide on how infectious diseases are spread, and how transmission levels can be controlled during an outbreak. 

What is an infectious disease?

An infectious disease is caused by pathogenic microorganisms such as bacteria, viruses, fungi or parasites. These microorganisms carry diseases and can be transmitted by direct contact, coming in contact with airborne particles or contaminated surfaces and food and insect bites. 

The transmission of the organism allows the new host’s body to become infected with one of these pathogens; this is how infection occurs within a human being’s system.

Some common infectious diseases include:

• Coronavirus (COVID-19)
• Common Cold
• Bird Flu
• Severe Acute Respiratory Syndrome (SARS)
• Influenza
• Measles
• Malaria
• Tuberculosis
• Ebola
• HIV/AIDS
• Hepatitis A, B and C
• Polio (Poliomyelitis)
• Dengue
• Yellow Fever
• Smallpox
• Whooping Cough (Pertussis)
• Tetanus

What are the ways infectious diseases are transmitted and spread?

An infectious disease, such as COVID-19, can spread because they are communicable diseases. Once they incubate in the initial host, they can be passed from person to person. 

This is how an infectious disease differs from a non-infectious disease, which is not communicable, meaning it can’t spread from person to person. Examples of non-infectious disease include cancer, diabetes and heart disease.

Infectious diseases can spread in the following ways:

Direct contact

One of the most common ways infectious diseases can be spread is through direct contact with an infected human or animal, or their bodily fluids. 

Person-to-person transmission

Humans are social creatures, and behaviours such as hugging, hand shaking and kissing can transmit an infectious disease. But while many infectious diseases are spread through direct physical contact, you don’t necessarily need to be touched by an infected person for transmission to occur. 

Spending an extended period of time around an infectious individual, such as a colleague or family member, can also cause indirect transmission via surfaces they touch, or droplets left in the air. 

Mother to an unborn child

A pregnant woman can pass on germs that can give her baby an infectious disease while still in the womb or during childbirth. Some infectious diseases that can be transmitted to a baby during gestation include hepatitis, syphilis, herpes, and HIV.

Animal to person transmission

Diseases that spread from an animal or insect to a human are known as zoonosis or zoonotic disease. Often these diseases don’t make the animal ill. The animal will serve as a host until transmission occurs to a human, whose immune system isn’t capable of defending against the disease. 

Zoonotic diseases can range in severity from some minor short-term illnesses to major life-altering illnesses.

Examples of zoonotic diseases include: 

• Animal flu
• Anthrax
• Bird flu
• Bovine tuberculosis
• Brucellosis

Airborne transmission

Coughing, sneezing and even talking can send droplets containing small particles of the disease into the air. If a non-infected person breathes in these particles, they may get sick too!

Contaminated surfaces

Surfaces can be contaminated with droplets from coughs or sneezes and can infect a person when they touch the surface with their hands. If they don’t wash their hands before touching their face, the disease could enter their body.

Vector-borne diseases

When we think of insect bites that cause disease, we often conjure images of mosquitos, but they are just one of many insects, also known as vectors, that spread disease.

A vector is a living organism with the ability to transmit infectious pathogens between humans, or from animals to humans. Common vectors are bloodsucking insects, which ingest disease-producing microorganisms from an infected host (animal or human) and later transmit it to a new host.

Vector parasites include:

• Mosquitos
• Aquatic snails
• Blackflies
• Fleas
• Lice
• Sandflies
• Ticks
• Triatomine bugs

Vector-borne diseases account for more than 17% of all infectious diseases and account for more than 700,000 deaths annually around the world. Common vector-borne diseases include Yellow Fever, Zika, Lyme disease and Chagas disease.

Contaminated food

Infectious diseases can also spread through contaminated food and water. The usual culprit is the bacteria E. coli, which is often transmitted through the improper handling of produce or undercooked meat.

Improperly canned food can also create the perfect environment for producing Clostridium botulinum, toxic spores that when ingested attacks the body’s nervous system leading to difficulty breathing, muscle paralysis, and even death.

Is it possible to manage the spread of infectious diseases?

Governments around the world have worked to manage and reduce the spread of infectious diseases, such as COVID-19, by implementing practices and restrictions for the safety of the community. These include:

Lockdown measures

Many countries have used lockdown measures as a means to reduce and prevent the spread of COVID-19, but this measure isn’t a new one. State-induced lockdowns can be traced back to the 14th century as a response to limiting the spread of the Black Death plague in Europe. 

The city of Athens is also known to have closed the port of Piraeus and sealed its population inside the city walls during the Peloponnesian War with Sparta to prevent plague from spreading across its Aegean network. 

Infectious diseases thrive on proximity. reducing the opportunities for groups of people to congregate, or travel, helps reduce the rate of transmission.

Physical distancing

One of the best ways to slow the spread of viruses is physical distancing. The more space between you and others, the harder it becomes for a virus to spread. 

The Australian Health Department has outlined several ways you can implement physical distancing and these include:

• Keeping 1.5 metres away from others wherever possible
• Avoiding physical greetings such as handshaking, hugs and kisses
• Practising extra care if you are using public transport
• Avoiding crowds and large public gatherings

Maintaining good hygiene

Hygiene practices reduce the risk of transmission by killing the germs and microbes that carry diseases. Soaps, sanitisers and other hygiene products contain ingredients and chemicals that are harmless to humans but kill these microorganisms. 

However, there is no guarantee it will work against every infection so be sure to maintain good personal hygiene habits as part of your daily life.

How you can help prevent and manage the spread of infectious diseases

The lives of children and families in Papua New Guinea are at risk because of a catastrophic surge in COVID-19 cases since mid-February 2021. People are becoming extremely ill with COVID-19 and many have already lost their lives to the disease. But the lives of children and families who can no longer access essential health services are also in danger.

You can help resupply health clinics and improve access to medical services by making a tax-deductible donation. Your gift will support the COVID-19 response in Papua New Guinea and help vulnerable families make it through the COVID-19 pandemic.