Bacterial and Viral Infections

Bacterial and viral infections commonly make people sick. Both are caused by microbes, microscopic organisms that easily enter human bodies and begin reproducing in people’s cells. Bacteria cause bacterial infections, while viruses cause viral infections. Each type of infection can cause a range of illnesses in a person. Some of these illnesses are relatively mild and usually resolve on their own, as in the case of common colds, which are caused by viruses. Other illnesses, such as strep throat and urinary tract infections (UTIs)—both caused by bacteria—can be more serious and may need to be treated with medication. Antibiotic drugs are taken for bacterial infections, while antiviral medications treat viral infections. Abuse of either type of drug can allow bacteria and viruses to become drug-resistant and more difficult to treat in the future. Many viral infections can be prevented altogether with vaccines.

Background

Both bacteria and viruses are microbes. Although both can cause infections in the bodies of humans and other animals, the two microbes are different in some fundamental ways. Bacteria are single-celled organisms that have existed for about 3.5 billion years. They can survive almost anywhere, including in extreme conditions. Many bacteria live inside the human body, where they execute useful functions, such as helping to digest food, helping fight harmful microorganisms, and producing essential vitamins. Bacteria are also used to make yogurt and fermented milk. Most bacteria—about 99 percent—are either helpful or harmless to the human body.

Other types of bacteria are pathogenic, meaning they can cause infections in other organisms. Pathogenic bacteria reproduce rapidly once they have entered a body. They then emit toxic chemicals that harm cells and ultimately lead to illnesses such as sinusitis, ear infections, strep throat, pneumonia, tuberculosis, Lyme disease, tetanus, and bacterial meningitis. Some bacterial infections are contagious and can be spread through respiratory droplets released through coughs and sneezes. Infected people can spread their bacterial germs on surfaces, food, and water. If uninfected people inadvertently touch these germs and transfer them to their mouth, eyes, or nose, they may become infected by the bacteria.

Viruses cause infections just as bacteria do, but in a different way. Viruses are extremely small organisms, smaller than the largest bacteria. Unlike bacteria, they cannot survive on their own but rather need living hosts in which to replicate themselves. A virus, composed of a simple capsule containing genetic information, invades a healthy cell and reprograms it to create copies of itself. This is how viruses spread within a host organism. Reprogrammed cells eventually die, causing sickness.

Viral infections are not as easy to treat as bacterial infections. Rounds of antibiotics may cure most bacterial infections, but they have no effect on viral infections because viruses essentially hide inside their host’s cells. This makes viruses more difficult to treat, as they replicate inside host cells, limiting the effectiveness of many medications. The most effective way to avoid becoming infected with viruses is by receiving vaccines, which can prevent illnesses such as polio, measles, chickenpox, and influenza (the flu). Antiviral medications can suppress viral replication and, in some cases, clear infections entirely. For example, direct-acting antivirals can cure hepatitis C, while antiviral treatments for human immunodeficiency virus (HIV), herpes, and influenza help manage and control the virus.

Impact

A commonly known type of bacterial infection is streptococcal pharyngitis, or strep throat. This illness involves an inflamed and sore throat, white spots on the tonsils, fever, and swollen lymph nodes in the neck. Oral antibiotics may be used to treat strep throat. Patients should exercise great care when taking antibiotics for any kind of bacterial infection. Bacteria are adaptable organisms that can become resistant to antibiotics. Over time, taking antibiotics unnecessarily can lead to antibiotic resistance as bacteria develop antibiotic resistance through genetic mutations. This can lead to the emergence of drug-resistant “superbugs” like MRSA (methicillin-resistant Staphylococcus aureus). A person who later contracts a bacterial infection from antibiotic-resistant bacteria will be unable mostly to treat the illness. This could lead to further health complications.

Another common bacterial infection is sinusitis, or a sinus infection. This type of infection inflames the tissues in the sinuses, causing a runny nose, cough, head congestion, and facial pain. Pneumonia is a bacterial infection (viruses or fungi can also be causal agents) that inflames the lungs while filling them with fluid. The bacterial infection tuberculosis causes a persistent, often blood-filled cough, while a urinary tract infection can damage the kidneys if left untreated.

In many cases, bacterial infections are secondary infections, meaning they occur soon after a viral infection has a chance to weaken a host’s immune system. Secondary infections can usually be identified if symptoms of the virus seem worse than they should or last longer than expected. Common secondary infections include sinusitis, pneumonia, and ear infections, which involve ear pain and fever.

As with bacterial infections, illnesses caused by viral infections range from mild to serious. The common cold—with its symptoms of a runny nose, cough, sneezing, sore throat, and head congestion—is caused by a virus. The influenza virus causes influenza, or the flu, which manifests all the symptoms of a cold but with the addition of a high fever and body aches. Annual vaccines can reduce the risk of people from contracting the flu during flu season; antiviral drugs can lessen the severity of the flu if they are taken within about two days of contracting the illness.

The viruses that cause less serious infections in the body, such as colds or flus, generally do not require any medical action to resolve, as the body’s immune system will eventually kill them. Other viruses can be more serious and even deadly. Human papillomavirus (HPV) infections, measles, certain types of hepatitis, and the debilitating disease polio are all caused by viruses but can be prevented with vaccines. The respiratory syncytial virus (RSV) can be a severe condition causing bronchiolitis and pneumonia and affecting infants younger than 6 months and premature infants, people with chronic heart and lung conditions, and older adults. In 2023, vaccines for respiratory syncytial virus (RSV) were approved for older adults, expanding prevention options for viral respiratory infections.

Vaccines are effective only if they are administered before contraction of the illness. For certain other viral infections, antiviral medications may be taken as needed to lessen the infections’ symptoms. Infections caused by flu, herpes simplex, and HIV may all be treated with antiviral drugs. Like antibiotics, however, antiviral medications taken over an extended period can lead to the development of drug-resistant viruses that may be more difficult to treat.

The COVID-19 pandemic, resulting from SARS-CoV-2, emerged in 2019, and the World Health Organization declared the end of the COVID-19 global public health emergency in 2023. The highly transmissible virus caused millions of deaths worldwide and prompted the rapid development of mRNA vaccines, monoclonal antibodies, and antiviral treatments like Paxlovid. Other viral infections in the twenty-first century, such as Avian Influenza (H5N1, H7N9, H5N8), highlight the risk of zoonotic transmission—when viruses jump from animals to humans. Understanding, monitoring, and research on these viruses are critical for future pandemic prevention. Other infections that threaten modern populations include Ebola, Zika virus, mpox (previously named monkeypox), and the West Nile virus.

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