8 Known Diseases Caused by Microbes

By Jennifer Stearns, Michael Surette

There is no shortage of microbes that cause disease; some are notable for the number of people they infect, and others for the nastiness of the infections they cause. Following is a list of 8 known diseases caused by microbes.


Ebola is one of the most lethal viruses to infect humans, with a mortality rate reaching 90 percent in some outbreaks. Ebola is one of two known filoviruses that cause severe hemorrhagic fever in humans. First identified in 1976 in the Congo near the Ebola River, infections by this filovirus have been confined to small outbreaks in isolated regions of central west Africa.

Fruit bats are considered the natural host, but contaminated wild meat may be the source of initial infection. The virus is spread through contact with bodily fluids of infected people or animals.

What makes Ebola so nasty is that the virus can be in a person unnoticed for around 21 days. But when symptoms begin to show, the disease progresses rapidly, starting with fever, fatigue, muscle pain, and headache, and ending with vomiting, diarrhea, internal bleeding, and often external bleeding.


Bacillus anthracis, which causes the disease known as anthrax, is a Gram-positive sporulating bacteria that gets its name from the distinct coal-like, black tissue that is shed from a skin infection (anthrakis is Greek for coal). It’s primarily associated with infections of grazing animals — spores can persist in the soil for decades.


How often do you hear someone complain about being sick a couple days with the flu? More often, this is probably just the common cold. The real flu is a serious illness with fever, headaches, and fatigue lasting for many days. The flu is caused by the influenza virus and occurs seasonally throughout the world — it’s always flu season somewhere!

The way that we keep track of influenza strains is by naming them for the different variants of two viral coat proteins — hemagglutinin (H) and neuraminidase (N) — to which numbers are assigned. The 1918 pandemic, as well as illness caused in 2009–2010, for example, were both caused by H1N1 viruses.


Tuberculosis (TB) is a global epidemic and a leading cause of morbidity and mortality throughout the world. Each year, it infects up to 9 million people and is responsible for over 1.5 million deaths.

Once infected with Mycobacterium tuberculosis, about 5 percent of people will develop active disease within five years; the remaining 95 percent will have a latent infection that can persist for decades, later becoming activated in about 5 percent of these individuals. With treatment, the mortality rate is less than 2 percent, but without treatment mortality rates are much higher.

A major change that has increased the severity of TB is the emergence of multidrug-resistant strains. These strains of M. tuberculosis are resistant to the antibiotics that were previously most effective against it, rifampicin and isoniazid. Extensively drug-resistant TB (XDR-TB) comes from strains that are resistant to all the most potent antibiotics, making them the most difficult to treat.

Almost half a million multidrug-resistant TB cases and an increasing number of XDR-TB strains are occurring each year, making an untreatable epidemic of TB a real possibility if new drugs are not discovered.


As its name suggests, the human immunodeficiency virus (HIV) infects immune cells. The immune suppression caused by HIV infection makes patients vulnerable to secondary infections from other bacteria, viruses, fungi, or protozoa.

It’s these secondary infections that cause problems for those infected with HIV — they become hard to treat as the patient’s immune system deteriorates. Acquired immunodeficiency syndrome (AIDS) applies to the most advanced stages of HIV infection where secondary infections are often fatal.

In 2012, it was estimated that 35 million people were living with HIV, with more than 2 million new infections and 1.6 million deaths per year. Secondary TB infections are attributed to about 25 percent of all HIV-related deaths. The virus is spread through sexual contact, bodily fluids, and contaminated needles.


Cholera is an acute diarrheal disease, caused by the waterborne bacterium Vibrio cholerae. Every year, it affects 3 million to 5 million people and leads to up to 200,000 deaths. The rapid dehydration can be lethal within hours, but it can be effectively treated with oral rehydration therapy.

Of course, rehydration therapy can be challenging because the disease is associated with poor sanitation and poor water quality. There have been 7 major cholera pandemics around the world in the last 200 years that have resulted in tens of millions of deaths.


The smallpox virus, Variola major, has been a significant pathogen throughout human history and has been responsible for major epidemics in the past. It’s particularly noteworthy, however, because it represents a remarkable success story in the eradication of infectious disease. A concerted international effort that started in the 1960s was declared successful in 1980. Smallpox was the first and only major infectious human disease to be eradicated from natural transmission.

Primary amoebic menigoencephalitis

Bacteria and viruses are most of what comes to mind when people think of nasty microbes, but there are many examples of other types of organisms that can be particularly nasty. Of the small number of opportunistic amoeba pathogens, Naegleria fowleri deserves special attention because it’s known as the “brain-eating amoeba.”

This free-living freshwater amoeba finds its way into the nasal passages of its host and then eats its way along the olfactory nerve and makes its way along the nerve fibers into the brain, where it feasts. It causes a condition called primary amoebic menigoencephalitis (PAM), a very rare disease but one that can affect any individual and is acquired from fresh water, including swimming pools.

Symptoms appear within a week of exposure and start out as the loss of the senses of smell and taste, followed by generic symptoms of headache, vomiting, and fever, and ending with confusion, hallucinations, and seizures. Infections with N. fowleri are almost always lethal; death occurs one to two weeks after the initial symptoms appear.