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Infectious Diseases

Understand everything from transmission to treatment.

You’ve probably become intimately familiar with the term infectious diseases over the past few years. Between the COVID-19 pandemic and the recent monkeypox outbreak, experts are learning more about these contagious illnesses than ever before, from transmission to treatment.

With more than seven billion people living around the globe, completely avoiding infectious diseases is just not possible. What is possible, however, is understanding these diseases so you can better protect yourself, the people you love, and your community.

Infectious diseases are extremely common and spread in a variety of ways. 

Amanda K Bailey

What are infectious diseases?

They are caused by microscopic organisms called pathogens, according to the World Health Organization (WHO). These include certain viruses, bacteria, parasites, and fungi. A few you might already have on your radar include the common cold, the flu, and COVID-19, which are all examples of viral diseases. Another big one is foodborne illness, like the stomach flu, which can be triggered by bacteria, viruses, parasites, and toxins that you consume via contaminated food or water.

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What are the main types of infectious diseases?

There are four main categories of pathogens that can cause infections. Here’s what to know about each one:

Viral infection

A virus is a sneaky little bugger whose goal is to get inside your body. When it does enter your body it finds a normal cell to infect. Once it’s inside the cell, the virus will create newly copied viruses—this is called replicating. The virus will then leave the cell, which can kill, damage, or change it, making you sick in the process, per the US National Library of Medicine. It’s important to note that antibiotics do not work to treat viruses.

One of the most recent viruses to enter human history is the SARS-CoV-2 virus—the novel coronavirus that caused the COVID-19 pandemic. But viruses are also responsible for everything from the common cold to HIV.

Bacterial infection

Bacteria are single-cell microorganisms that come in all different shapes and sizes and exist all over the globe. One of the most interesting things about bacteria is that they aren’t just infectious organisms, per the National Institutes of Health (NIH). In fact, you have trillions of bacteria living in your body and on your skin, helping you with everyday processes like digestion, per the NIH. That means having plenty of good bacteria in and on your body is pretty vital to your functioning. But there are some bacteria that can make you seriously sick.

One big culprit of bacterial illness is the streptococcus family of bacteria, which causes strep throat, per the Centers for Disease Control and Prevention (CDC). Many bacteria, such as salmonella, are also responsible for foodborne illnesses.

Fungal infection

Fungi are spore-producing organisms, and just like bacteria, not all species cause diseases. But of the few hundred types of fungi that can make humans sick, they can cause everything from infections in the lungs to the bloodstream, per the CDC. If you’ve ever had a frustrating run-in with vaginal candidiasis—also known as a yeast infection—then you’ve had a fungal infection.

Parasitic infection

Parasites are organisms that live on or in other living things—called hosts—and often depend on them for their survival. Protozoa, helminths, and ectoparasites are the three types of parasites that can cause infections in people, per the CDC.

Giardiasis is an example of a protozoal infection, while tapeworm and roundworm infections are the results of helminths. Lice, ticks, and mosquitoes are all examples of common ectoparasites, some of which can transmit pathogens that can cause potentially serious infections like Lyme disease and malaria.

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How do infectious diseases spread?

An infectious disease that is spread from person to person is called a communicable disease. The spread of a disease from one person to another is called human-to-human transmission, but there is no shortage of ways in which infectious diseases can be spread. Here are the main ones:

  • Direct contact: This is when germs are spread through close contact between people, which can include activities like kissing, sex, or being in a very crowded space where your body is touching others.
  • Respiratory droplets: Germs that are released into the air when an infected person speaks, coughs, or sneezes can take up residence in the throat and nasal passages of another person.
  • Contaminated objects: Germs can easily remain on objects well after a sick person has touched them. Touching one of these contaminated objects and then touching your eyes, nose, or mouth with unwashed hands can make you sick. If you’ve ever seen someone disinfecting door handles or shopping carts during flu season, this is why.
  • Environment: Environmental exposure to pathogens occurs when you encounter germs from the air or your surroundings—so, not just from human respiratory droplets, but also from contaminated soil, water, or even food.
  • Skin injuries: You can get an infection if you have an injury that breaks the barrier between the skin and the bloodstream. For example, HIV and hepatitis C are two diseases that can be spread through contaminated needles.
  • Animal contact: When an animal carries an infectious disease, it can sometimes transmit that pathogen to humans. A disease that can jump from an animal to a human (and vice versa) is known as a zoonotic disease. Avian flu is an example of a virus that can spread from infected birds to humans.

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Are there universal symptoms of infectious diseases?

When it comes to infectious diseases, all symptoms are not created equal. That’s because your symptoms will largely depend on the pathogen that causes the disease, as well as the specific disease you’re fighting off and the part of your body it’s primarily affecting. For instance, a cold is an upper respiratory infection, meaning it tends to target your nose, sinuses, throat, and lungs, causing symptoms like coughing and congestion. A gastrointestinal infection will cause cramping and diarrhea because it affects your bowels.

Still, there are some “baseline” symptoms of infection that tend to appear across illnesses, as the body’s immune system kicks into gear. These symptoms can include fever, body aches, chills, sweating, and fatigue, per the CDC.

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How are infectious diseases treated?

It’s important to note that vaccines exist for a number of infectious diseases, and they can either help prevent you from getting sick and spreading the disease to others, or help reduce the severity of your illness should you still become infected.

As a general rule of thumb, bacterial infections are treated with antibiotics, fungal infections are treated with antifungals, and viral infections can sometimes be managed with antiviral medications, but symptoms are often managed at home as the virus runs its course. Parasitic infections are sometimes treated with antiparasitic medications but may go away on their own in some cases.1

Treatment for infections often comes down to managing your symptoms as well, which could involve taking pain relievers, antidiarrheal medications, decongestants, or cough suppressants (depending on what kind of infection you’re fighting). None of these things will eliminate the infection, but they’ll make the symptoms more bearable while your body works to get a handle on the germs.

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What are the most common infectious diseases?

There are so many different infectious diseases, it’s impossible to list all of them, but there are some relatively common ones to know about.

Cold and flu

Although the common cold and flu are often lumped into the same general category, there are some differences between the two conditions, which are primarily spread via respiratory droplets and close personal contact. Both infections are caused by viruses; the flu is specifically caused by the influenza virus, while a cold can be caused by any number of upper respiratory viruses. The common cold is not usually dangerous, but the flu can cause severe complications in young children, people with certain medical conditions that compromise the immune system, pregnant people, and older folks.


COVID-19 is an infection caused by the novel SARS-CoV-2 virus. It’s one of many coronaviruses, which are known for having spike proteins on their outer layer. While there are many different coronaviruses circulating, the SARS-CoV-2 virus emerged in late 2019, and spread rapidly around the world. Since then, it has mutated several times, spurring different, highly infectious variants of the original virus, and that has contributed to waves of infections since the original outbreak. This virus is primarily spread via respiratory droplets.

Many people will have mild symptoms when infected with COVID-19, but other people with severe symptoms may require hospitalization, especially those who are older or have compromised immune systems. COVID-19 can also cause long-term symptoms, known as long COVID, in some people; the infection has led to more than 1 million deaths in the US alone.

Foodborne illness

Foodborne illness, or food poisoning, can come with some seriously uncomfortable symptoms (vomiting and diarrhea rank pretty high). Everything from viruses, bacteria, and parasites to toxins and chemicals—that may find their way into food and water—can cause foodborne illness. You can get food poisoning by eating contaminated foods or coming into contact with the vomit or stool of someone who is infected. Norovirus, salmonella, Clostridium perfringens, campylobacter, and Staphylococcus aureus are the most common types of germs that cause food poisoning in the US, according to the CDC.


Pneumonia is an infection in the air sacs (alveoli) of the lungs. When they become inflamed, it leads to a buildup of fluid or pus. While pneumonia is usually viral or bacterial, it can also develop when you breathe in harmful chemicals. The viral and bacterial types primarily spread through respiratory droplets. Symptoms can vary depending on the type and severity of your infection, but bacterial pneumonia tends to be more serious.

Sexually transmitted infections (STIs)

STIs are infections transmitted between humans through sexual contact, which includes vaginal, anal, and oral sex. STIs are among the top infectious diseases in the world; in fact, more than 1 million people worldwide contract an STI every single day, according to the WHO.

The four most common bacterial STIs include syphilis, gonorrhea, chlamydia, and trichomoniasis, and are generally treated with antibiotics, though, some of these strains of bacteria are becoming antibiotic-resistant. The four most common viral STIs include hepatitis B, herpes, HIV, and human papillomavirus (HPV). These are usually managed with long-term antiviral therapy.

Hepatitis C

This is a viral illness that causes severe inflammation of the liver. It’s spread through exposure to blood infected with the hepatitis C virus and can cause both short-term and long-term illnesses. About 30% of people will clear the virus within six months after the initial infection, but chronic hepatitis C can last for years if not treated. Roughly 80% of people who contract hepatitis C do not have symptoms, according to the WHO. Even if you don’t have symptoms, you can develop a long-term infection. It’s crucial to get treatment because, without it, the condition can eventually cause liver damage.


Human immunodeficiency virus (HIV) is a viral infection that primarily spreads when you come into direct contact with the blood, semen, vaginal secretions, or anal fluids of someone with a detectable viral load, often through sexual contact or sharing syringes. HIV attacks and weakens the human immune system, increasing the risk for other infections or conditions to develop. Acquired immunodeficiency syndrome (AIDS) is an advanced form of HIV infection that develops over many years if the initial infection is not treated.

HIV is a highly stigmatized disease, which can make living with it emotionally challenging. However, we now have highly effective treatments that keep HIV from progressing. Antiretroviral therapy (ART) is a specific combination of antiviral drugs that help prevent the virus from replicating. After six months of treatment, ART is able to bring a person’s viral load down to an undetectable level—which means that they can no longer transmit the virus.


Tuberculosis (TB) is a bacterial infection that is caused by mycobacterium tuberculosis and spreads through respiratory droplets. It typically attacks the lungs, but can also go after the kidneys, spine, and brain. TB can be either an active infection or the bacteria can be latent, meaning it hangs out in the body without causing any kind of infection. Because TB can be fatal if left untreated, an active TB infection should be treated right away.


Malaria is a serious parasitic infection, spread from infected mosquitoes to humans. Four African regions are disproportionately affected by worldwide malaria infections and deaths. If left untreated, severe cases can be fatal within 24 hours. The first symptoms—fever, headache, and chills—may not be immediately recognized as malaria, but the WHO stresses that early diagnosis and treatment can both prevent death and limit the spread.

Lyme disease

Lyme disease is a bacterial infection that’s spread through the bite of an infected tick. In most cases, the tick has to be attached to your body for 36 to 48 hours to spread the disease. Lyme disease may cause a bull’s-eye-shaped rash, fever, chills, headache, joint pain, and fatigue, as well as complications like facial paralysis if left untreated. That’s why it’s important to diagnose and begin treatment for Lyme disease as early as possible. Antibiotics are used to treat the infection, but some people report long-term symptoms, even with treatment.


Monkeypox is another viral infection, which is caused by the same family of viruses that cause smallpox. Although monkeypox has been around since 1958—the first documented human case occurred in 1970—a resurgence of the virus in non-endemic countries is currently being tracked by the CDC. It’s spread through close personal contact, like touching the rash or scab from the rash. One of the telltale symptoms of monkeypox is a painful, itchy, or blistery rash that can appear anywhere on the body.

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How to prevent the spread of infectious diseases

It’s important to take steps to prevent the spread of infectious diseases, for yourself and others. Here are some good practices to get in the habit of doing:

  • Get vaccinated, if you can.
  • Wash your hands regularly.
  • Cover your coughs and sneezes with a tissue, or the crook of your elbow.
  • Stay home when you’re sick.
  • Avoid close contact with people who are sick.
  • Practice safe sex and use protection, like a condom.
  • Get tested for STIs regularly.
  • Clean and disinfect high-touch surfaces.
  • Be mindful of good food safety practices.
  • Only take antibiotics as prescribed.

This isn’t an exhaustive list, but anything you can do to prevent the spread of these diseases ultimately benefits you, your family, and your community.


  1. JAMA, Medications for Treating Infection
  2. National Library of Medicine, What You Need to Know About Infectious Diseases

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