Bacteria are the smallest forms of life. Biologists believe that bacteria are a separate life form — they are different from plants and animals. Bacteria cause most of the infections in people with cancer. Some bacteria that commonly cause infections in people with cancer include:
- Pseudomonas aeruginosa
- Klebsiella pneumonia
- Escherichia coli (E. coli)
- Clostridium difficile (C. diff)
- Staphylococcus aureus (Staph aureus)
- Staphylococcus epidemidis (Staph epi)
- Streptococcus viridans
Viruses are the smallest known germs. Unlike bacteria, they are not really alive because they cannot grow on their own. Viruses can only make new viruses when they are inside living cells, such as human, animal, or plant cells. Most viral infections in people with very low white blood cell counts are caused by
- Varicella zoster virus (VZV), the virus that causes chickenpox and shingles
- Herpes simplex virus (HSV), the virus that causes cold sores and genital herpes
- Cytomegalovirus (CMV)
Other viruses, such as respiratory and hepatitis viruses, may cause problems, too.
Varicella zoster virus
Varicella zoster virus (VZV) can cause serious infections in people with cancer, especially children. Unlike many other infections, a VZV infection never completely goes away even in a healthy person. This means when a person recovers from chickenpox, some of the virus stays in their nerve cells. If the person’s immune system is weakened, even many years later, the virus can become active again and cause a problem known as shingles. People with shingles have groups of tiny, painful blisters on their skin. The blisters form along the paths of nerves. The pain from shingles can last long after the blisters go away.
These skin blisters hurt, but the most serious part of VZV infection in people with a weak immune system is that the virus may spread to other organs. This can lead to pneumonia (lung infection) or encephalitis (infection of the brain). There’s a high risk of serious damage from VZV in people with low white blood cell counts and weak immune systems. Unlike chickenpox infections in healthy people, VZV infections can be deadly in people with cancer.
Herpes simplex virus
Herpes simplex virus (HSV) is from the same family of viruses as varicella zoster. Like varicella zoster, HSV causes mild infections in people with healthy immune systems, but it also stays in their nerve cells. It can become active again years later, especially if the immune system changes. And like varicella zoster, HSV can also cause pneumonia and encephalitis.
Cytomegalovirus (CMV) infection is common in healthy people and it’s usually not serious. Many people have the virus in their bodies for years without even knowing it. But when the immune system is weakened, CMV can cause things like serious pneumonia, enteritis (intestinal infection), hepatitis (liver infection), and retinitis (a serious eye infection that can lead to blindness if not treated).
CMV infection can be very hard to treat in people with low white blood cell (WBC) counts, because the drugs that work against the virus also lower the number of WBCs. This makes it hard for the body to fight the infection. Often, the best thing to do for patients with weak immune systems is to try to prevent the infection from flaring up. This is done by giving patients certain anti-viral drugs before their symptoms begin.
Respiratory viruses are those like influenza (the flu), respiratory syncytial virus (RSV), and other seasonal viruses. They can cause illnesses in people with normal immune systems, but these illnesses may become more severe in those with weak immunity. Respiratory viruses can affect the nose, throat, sinuses, breathing passages, and lungs. Pneumonia, which affects the lungs, is the most serious problem that can be caused by respiratory viruses. Pneumonia is more likely when one of these viruses infects a person whose immune system isn’t working well.
The best way to prevent the flu is to get a flu shot every fall and have other household members vaccinated, too. Wash your hands often when viral infections are going around to help decrease the chances of infection. People with low white blood cell counts should try to stay away from crowds and people with these kinds of infections. (See the section called “What can people with cancer do to prevent infections?”)
Protozoa are one-celled creatures thought to be the smallest and simplest form of animals. Some protozoa can infect people who have healthy immune systems. But these infections are more common in less-developed countries than in the United States. In the US, protozoa mostly cause disease in people with weak immune systems. People who have organ transplants, cancer, AIDS, or other diseases can get life-threatening infections with protozoa. Common protozoa that can cause serious illness in people with cancer include
- Toxoplasma gondii
Toxoplasma gondii is found in soil, cat waste, water contaminated with cat waste, and undercooked meats. It can cause fever and lymph node swelling or no symptoms at all in adults with normal immune systems. It usually stays inactive in healthy people, but when the immune system is weak the infection may become active and damage the brain or heart. People with cancer can have old infections become active again, or they can get infected for the first time while their immune system is weak.
Cryptosporidium is a common cause of diarrhea and stomach pain in people with weak immune systems. It’s spread by infected people and animals, often through drinking water or produce contaminated with stool. It can cause very severe diarrhea, leading to malnutrition, weight loss, imbalances in blood chemistry, and dehydration (severe fluid loss).
In humans, fungi can live in balance with other germs that normally live on or in the body without causing symptoms or damage. But fungal infections can happen when there are changes in this balanced environment. Things that can change the normal balance include:
- Damage to the skin or mucous membranes
- Low white blood cell counts
- A weak immune system
- Fewer bacteria than normal on the body’s surfaces or mucous membranes (such as the intestines or vagina — this often happens with antibiotic treatment)
Fungal infections can be serious and even deadly. Fungi that commonly infect people with cancer include:
- Pneumocystis jirovecii (formerly known as P. carinii)
- Candida (yeast)
Pneumocystis jirovecii is an atypical fungus that can cause illness in those with weak immune systems. It causes pneumonia and rarely spreads to other organs, but the pneumonia can make it very hard to breathe. Pneumocystis infections are common enough in patients with weak immune systems that sometimes doctors will give an antibiotic to help prevent this illness.
This is the most common fungal infection. Candida can live in a healthy person without causing any problems. Sometimes it may cause a mild skin rash or vaginal discharge (called a yeast infection). In babies, it can cause a mouth infection called thrush. But a person with a weak immune system is at risk for a more serious version of thrush. It can affect the mouth and esophagus (swallowing tube) and may spread to other organs. Candida can also cause bloodstream infections in people with weak immune systems.
Aspergillus is a fungus that’s often found in the air and in our environment. It’s rarely a problem in healthy people, but it can cause a mild lung infection in people with allergies or asthma. In people with cancer, though, it can cause serious infections of the sinuses, lungs, kidneys, brain, and heart valves in people. This is especially true for those with very low white blood cell counts or those getting cancer treatments that suppress the immune system. This type of infection is often hard to diagnose. Quick, aggressive treatment is needed as soon as it’s suspected.
Cryptococcus is found in the soil and in bird droppings, especially pigeon waste. It’s thought to be spread by breathing in the germ after it’s dried out and gets stirred up into the air. In people with healthy immune systems it may cause a lung infection that goes away without symptoms. But the fungus can remain inactive in the lungs for years. And if the person’s immune system becomes weak, Cryptococcus can begin to grow and spread to other parts of the body. One of the most serious outcomes of this infection is meningitis, an infection of the membranes that cover the brain and spinal cord.
Histoplasma is another fungus that often infects the lungs of healthy people without causing symptoms or tissue damage. Infection with Histoplasma is quite common in the Mississippi and Ohio River Valleys in the United States, although it is seen outside of these regions. It’s also found in many other parts of the world. People become infected through contact with soil or breathing the dust from soil that contains bird or bat waste. Like Cryptococcus, the fungus may remain inactive for years in the lungs of healthy people. But it can become active if their immune system is weakened. In people with cancer, Histoplasma may cause a serious illness and may spread to the lymph system, liver, spleen, and other organs.
Anyone who has lived in any part of Arkansas, Kentucky, Missouri, Tennessee, and West Virginia is likely to be infected. People from certain parts of Alabama, Illinois, Indiana, Iowa, Kansas, Louisiana, Maryland, Mississippi, Nebraska, Ohio, Oklahoma, Texas, and Virginia are also likely to be infected with Histoplasma.
Coccidioides causes a fungal disease called coccidioidomycosis or Valley Fever. The fungus lives in the soil in the southwestern United States, parts of Mexico, and Central and South America. People breathe in this fungus when dust containing it is stirred up. Most people with healthy immune systems don’t know they have the disease and it goes away on its own. But Coccidioides can cause serious illness in people with weak immune systems. It can spread outside the lungs to the skin, nerves, brain, bones, and joints.
Last Revised: 11/06/2013