H1N1, also known as the swine flu virus, may be on our minds at the moment, but it is hardly the first pathogen to trigger fear and panic. From the avian flu to the bubonic plague, disease outbreaks have been a constant in human history. Scientists are locked in a perpetual struggle to ward off the latest and most virulent forms of pathogenic bacteria, viruses, and protists.
Swine flu was first diagnosed in the U.S. by the CDC in San Diego County, California, on April 14 of this year. The virus, the result of a reassortment of four strains of influenza A virus subtype H1N1 (one endemic in humans, one in birds, and two in pigs), has claimed around a hundred lives and been reported in over 50 countries.
C. jejuni and C. coli are microaerophilic bacteria that have a characteristic corkscrew shape. Both belong to the same genus, Campylobacter (which means "twisted bacteria"). They are the foodborne pathogens responsible for the campylobacteriosis infection, which can cause periodontitis, dysentery, and inflammatory diarrhea.
Scientists recently discovered that C. jejuni and C. coli are beginning to merge into one species through a process called hybridization. Though they share about 85 percent of their genetic code, the two species have traditionally been very different, having adapted to fill specific niches inside the guts of chicken, cows, and other livestock.
But because of industrialized farming--which involves keeping livestock in ultra-close proximity--C. jejuni and C. coli have been pushed closer together, facilitating the exchange of genes through a process called hybridization.
This false-color electron-microscope image shows Campylobacter cells clumping together.
Listeria monocytogenes, a rod-shaped bacterium, is one of the world's deadliest foodborne pathogens. It causes listeriosis, a group of life-threatening diseases that include meningitis, encephalitis, pneumonia, septicemia, and intrauterine or cervical infections in pregnant women. The latter account for around 27 percent of the 500 listeria deaths that occur in the U.S. every year.
L. monocytogenes infects and moves through white blood cells using actin rockets, also known as "comet tails." The rockets work like this: A protein anchored to the bacterium's membrane triggers the rapid polymerization of the protein actin; this provides an explosive boost, so the bacterium can push through the membrane of white blood cells and burst out to infect another cell.
In this image, the bacteria (shown in red) are traveling around a cell using their bright actin rockets.
Streptococcus pyogenes, a spherical bacteria that typically grows in long chains, can cause minor infections like impetigo to potentially deadly diseases like streptococcal toxic shock syndrome. The cells possess a number of defense mechanisms, known as virulence factors, which allow them to evade the host's immune system and spread through its tissues.
In this image, human neutrophils--white blood cells that are one of the body's first lines of defense--are engulfing S. pyogenes cells through a process known as phagocytosis.
Giardia lamblia, a human parasite found in contaminated drinking water, is responsible for over 20,000 intestinal infections a year in the U.S. A single-celled eukaryote, Giardia enters the body in unfiltered water or I transmitted through person-to-person contact. It eventually makes its way to the intestine, where it lives as a trophozoite, and undergoes asexual reproduction.
Symptoms of giardiasis include abdominal cramps, vomiting, fever, and diarrhea. Once it is expelled from the body through stool, it assumes the form of an infectious cyst that can survive in water, soil, or on surfaces for weeks.
In this image, the trophozoites' microtubule skeletons are immunostained (red), while their paired nuclei are DAPI stained (fluorescent blue).
Named after the Ebola River Valley in the Democratic Republic of Congo, where the first outbreak occurred, this deadly hemorrhagic fever is caused by a group of viruses (shown here) that belong to the genus Ebolavirus.
Though mostly confined to African countries, the disease came to the United States in 1989 when a new strain was discovered in crab-eating macaques imported from the Philippines to Reston, Virginia. Several outbreaks caused by the Reston strain have occurred in Texas, Pennsylvania, and Italy. Of the 1,850 or so patients that have been afflicted by the disease since it was first discovered, over 1,200 have died.
The virus attacks the host's endothelial cells and platelet cells, damaging both and preventing the latter from coagulating. As a result, victims lose large amounts of blood and eventually collapse and die from shock.
More commonly known as human herpesvirus 5 or HCMV, cytomegalovirus is the most frequently transmitted intrauterine infection and can result in serious disabilities at birth, though most people will exhibit only mild symptoms or none at all.
About 1 in 150 children is born with congenital CMV, and 1 in 750 children suffers from permanent disabilities due to the infection. This multicolor immunofluorescence image shows human endothelial cells being infected by cytomegalovirus.
Nematocida parisii, a newly discovered species of the protozoan parasites known as microsporidia, lives in the intestines of C. elegans, a small roundworm commonly used for research.
Microsporidia can be found in water supplies and can cause a variety of infections, known as microsporidiosis, that affect people with weakened immune systems. The parasites can infect the lungs, kidney, brain, muscles, eyes, sinuses, or intestines, and can cause gallbladder disease and a debilitating array of gastrointestinal symptoms, including chronic diarrhea and wasting.
This image shows a section of roundworm intestine stained to show N. parisii's cytoplasm in red and DNA in blue.