Blood infections fall into four broad categories based on the type of organism involved: bacterial, fungal, viral, and parasitic. Bacterial infections account for the vast majority of cases, but within each category are dozens of specific pathogens, and the way these infections behave in the body ranges from harmless and temporary to life-threatening. Understanding the different types helps clarify why some blood infections resolve on their own while others become medical emergencies.
The Four Categories of Blood Infection
The simplest way to classify blood infections is by what’s causing them. Bacteria are by far the most common culprits, but fungi, viruses, and parasites can all enter and circulate through the bloodstream.
- Bacterial bloodstream infections are the most frequently diagnosed type. The two most common bacteria found in blood cultures are Staphylococcus aureus and E. coli, which together account for over 40% of all bacterial bloodstream infections worldwide. Other frequent offenders include Klebsiella pneumoniae (7.7% of cases), Pseudomonas aeruginosa (5.3%), and Enterococcus faecalis (5.2%).
- Fungal bloodstream infections are less common but serious. The main one is candidemia, caused by Candida yeast species. Roughly 25,000 cases occur in the United States each year. Five species of Candida cause up to 95% of these infections, with Candida albicans being the leading cause overall.
- Viral bloodstream infections (called viremia) involve viruses circulating in the blood. Examples include cytomegalovirus (CMV), hepatitis B, HIV, herpes simplex virus, and adenovirus. These are particularly dangerous in people with weakened immune systems.
- Parasitic bloodstream infections involve protozoa or worms that enter the bloodstream. Malaria, caused by Plasmodium parasites transmitted through mosquito bites, is the most well-known example globally. Others include Chagas disease (caused by Trypanosoma cruzi) and babesiosis.
Bacteremia, Sepsis, and Septic Shock
Beyond the type of organism, blood infections are also classified by severity. This distinction matters more for your health than which specific bug is involved, because it determines how urgently the infection needs treatment and how dangerous it is.
Bacteremia simply means bacteria are present in the bloodstream. This can happen from something as routine as vigorous toothbrushing. Most of the time, only a small number of bacteria enter the blood, your immune system clears them quickly, and you never notice. Bacteremia becomes a problem when the immune system can’t keep up, allowing bacteria to multiply or spread to organs.
Sepsis is what happens when the body’s response to a blood infection spirals out of control. Rather than fighting the infection in a targeted way, the immune system triggers widespread inflammation that starts damaging the body’s own organs. Sepsis typically causes fever, weakness, rapid heart rate, and fast breathing. The kidneys, heart, and lungs can begin to malfunction. Mortality rates for sepsis range from 15% to 25% in high-income countries and frequently exceed 40% in lower-income settings.
Septic shock is the most dangerous stage. Blood pressure drops to dangerously low levels, and organs stop receiving enough blood to function. Mortality from septic shock reaches 30% to 40% in well-resourced hospitals and can exceed 50% elsewhere.
Where Blood Infections Start
Most blood infections don’t begin in the blood itself. They start as infections elsewhere in the body that eventually spread into the bloodstream. The most common starting points are urinary tract infections, pneumonia, surgical site infections, and infections around intravenous catheters or other medical devices. In hospitals, catheter-related bloodstream infections are a major concern because the tubing creates a direct path for bacteria to enter the blood.
Community-acquired bloodstream infections (those picked up outside a hospital) are dominated by E. coli and Staphylococcus aureus, which together cause nearly 58% of cases. Hospital-acquired infections are more varied, with a wider range of bacteria involved, many of which are more resistant to standard treatments.
How Bacterial Patterns Are Shifting
The landscape of blood infections has changed over the past two decades. Data from a 20-year global surveillance program published in Antimicrobial Agents and Chemotherapy shows that E. coli has overtaken Staphylococcus aureus as the most common bloodstream pathogen. In the late 1990s, Staph aureus made up 22.5% of bloodstream infections; by 2013 to 2016, it had dropped to 18.7%. Over the same period, E. coli rose from 18.7% to 24.0%. Klebsiella pneumoniae infections also increased across all regions, with the sharpest rises in Europe and the Asia-Pacific region. These shifts reflect changing patterns in antibiotic use, aging populations, and the spread of drug-resistant strains.
How Blood Infections Are Detected
The primary tool for identifying a blood infection is a blood culture. Doctors draw blood from a vein, place it in special bottles that encourage microbial growth, and wait to see what grows. The standard practice is to collect two to four separate sets of blood cultures within a 24-hour period, with each set drawn from a different vein. This matters because a single positive culture could represent contamination from skin bacteria during the draw. Two or more positive cultures from different sites provide much stronger evidence of a true infection.
Each set typically includes one bottle designed to grow organisms that need oxygen and one for organisms that thrive without it. The target is about 20 milliliters of blood per set, for a total of 40 to 60 milliliters across all sets. Ideally, blood is collected before any antibiotics are given, since antibiotics in the blood can prevent bacteria from growing in the culture and lead to a false negative result.
Why Speed of Treatment Matters
Once a blood infection is suspected, particularly when sepsis is a concern, the speed of antibiotic delivery directly affects survival. Delays in getting antibiotics into the bloodstream are associated with increased mortality. Research estimates that roughly one-third of the total delay between a patient arriving at the hospital and receiving antibiotics happens after the doctor has already placed the order, during the time it takes for the pharmacy to prepare and deliver the medication. Many hospitals now stock common antibiotics directly in emergency departments and intensive care units to cut this gap.
For fungal bloodstream infections like candidemia, antifungal medications are used instead. Viral blood infections are treated with antiviral drugs when effective ones exist, though for some viruses the approach focuses more on supporting the immune system. Parasitic blood infections each have their own targeted treatments depending on the specific organism involved.