Infection

Infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Infections are caused by infectious agents including viruses, viroids, prions, bacteria, nematodes such as parasitic roundworms and pinworms, arthropods such as ticks, mites, fleas, and lice, fungi such as ringworm, and other macroparasites such as tapeworms and other helminths.

Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.

Specific medications used to treat infections include antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics. Infectious diseases resulted in 9.2 million deaths in 2013 (about 17% of all deaths). The branch of medicine that focuses on infections is referred to as infectious disease.

(Wikipedia)

Classification

Subclinical versus clinical (latent versus apparent)

Symptomatic infections are apparent and clinical, whereas an infection that is active but does not produce noticeable symptoms may be called inapparent, silent, subclinical, or occult. An infection that is inactive or dormant is called a latent infection.^[6] An example of a latent bacterial infection is latent tuberculosis. Some viral infections can also be latent, examples of latent viral infections are any of those from the Herpesviridae family.

The word infection can denote any presence of a particular pathogen at all (no matter how little) but also is often used in a sense implying a clinically apparent infection (in other words, a case of infectious disease). This fact occasionally creates some ambiguity or prompts some usage discussion. To get around the usage annoyance, it is common for health professionals to speak of colonization (rather than infection) when they mean that some of the pathogens are present but that no clinically apparent infection (no disease) is present.

A short-term infection is an acute infection. A long-term infection is a chronic infection. Infections can be further classified by causative agent (bacterial, viral, fungal, parasitic), and by the presence or absence of systemic symptoms (sepsis).

Infectious Mononucleosis

Symptoms

Each infectious disease has its own specific signs and symptoms. General signs and symptoms common to a number of infectious diseases include:

Fever

Diarrhea

Fatigue

Muscle aches

Coughing

When to see a doctor

Seek medical attention if you:

Have been bitten by an animal

Are having trouble breathing

Have been coughing for more than a week

Have severe headache with fever

Experience a rash or swelling

Have unexplained or prolonged fever

Have sudden vision problems

Causes

Infectious diseases can be caused by:

• Bacteria. These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.
• Viruses. Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to AIDS.
• Fungi. Many skin diseases, such as ringworm and athlete's foot, are caused by fungi. Other types of fungi can infect your lungs or nervous system.
• Parasites. Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be transmitted to humans from animal feces.

Direct contact

An easy way to catch most infectious diseases is by coming in contact with a person or animal who has the infection. Three ways infectious diseases can be spread through direct contact are:

• Person to person. A common way for infectious diseases to spread is through the direct transfer of bacteria, viruses or other germs from one person to another. This can occur when an individual with the bacterium or virus touches, kisses, or coughs or sneezes on someone who isn't infected.
  These germs can also spread through the exchange of body fluids from sexual contact. The person who passes the germ may have no symptoms of the disease, but may simply be a carrier.
• Animal to person. Being bitten or scratched by an infected animal — even a pet — can make you sick and, in extreme circumstances, can be fatal. Handling animal waste can be hazardous, too. For example, you can acquire a toxoplasmosis infection by scooping your cat's litter box.
• Mother to unborn child. A pregnant woman may pass germs that cause infectious diseases to her unborn baby. Some germs can pass through the placenta. Germs in the vagina can be transmitted to the baby during birth.

Indirect contact

Disease-causing organisms also can be passed by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle.

When you touch a doorknob handled by someone ill with the flu or a cold, for example, you can pick up the germs he or she left behind. If you then touch your eyes, mouth or nose before washing your hands, you may become infected.

Insect bites

Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks — to move from host to host. These carriers are known as vectors. Mosquitoes can carry the malaria parasite or West Nile virus, and deer ticks may carry the bacterium that causes Lyme disease.

Food contamination

Another way disease-causing germs can infect you is through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. E. coli, for example, is a bacterium present in or on certain foods — such as undercooked hamburger or unpasteurized fruit juice.

Risk factors

While anyone can catch infectious diseases, you may be more likely to get sick if your immune system isn't working properly. This may occur if:

• You're taking steroids or other medications that suppress your immune system, such as anti-rejection drugs for a transplanted organ
• You have HIV or AIDS
• You have certain types of cancer or other disorders that affect your immune system

In addition, certain other medical conditions may predispose you to infection, including implanted medical devices, malnutrition and extremes of age, among others.

Complications

Most infectious diseases have only minor complications. But some infections — such as pneumonia, AIDS and meningitis — can become life-threatening. A few types of infections have been linked to a long-term increased risk of cancer:

• Human papillomavirus is linked to cervical cancer
• Helicobacter pylori is linked to stomach cancer and peptic ulcers
• Hepatitis B and C have been linked to liver cancer

In addition, some infectious diseases may become silent, only to appear again in the future — sometimes even decades later. For example, someone who's had a chickenpox infection may develop shingles much later in life.

Prevention

Infectious agents can enter your body through:

• Skin contact or injuries
• Inhalation of airborne germs
• Ingestion of contaminated food or water
• Tick or mosquito bites
• Sexual contact

Follow these tips to decrease your risk of infecting yourself or others:

• Wash your hands. This is especially important before and after preparing food, before eating, and after using the toilet. And try not to touch your eyes, nose or mouth with your hands, as that's a common way germs enter the body.
• Get vaccinated. Immunization can drastically reduce your chances of contracting many diseases. Make sure to keep up to date on your recommended vaccinations, as well as your children's.
• Stay home when ill. Don't go to work if you are vomiting, have diarrhea or have a fever. Don't send your child to school if he or she has these signs and symptoms, either.
• Prepare food safely. Keep counters and other kitchen surfaces clean when preparing meals. Cook foods to the proper temperature using a food thermometer to check for doneness. For ground meats, that means at least 160 F (71 C); for poultry, 165 F (74 C); and for most other meat, at least 145 F (63 C).
  In addition, promptly refrigerate leftovers — don't let cooked foods remain at room temperature for extended periods of time.
• Practice safe sex. Always use condoms if you or your partner has a history of sexually transmitted infections or high-risk behavior.
• Don't share personal items. Use your own toothbrush, comb and razor. Avoid sharing drinking glasses or dining utensils.
• Travel wisely. If you're traveling out of the country, talk to your doctor about any special vaccinations — such as yellow fever, cholera, hepatitis A or B, or typhoid fever — you may need.

Worst Infections

1. Anthrax

Anthrax is a potential bioweapon, according to Dr. Tosh, notable not because of the number of deaths it causes but because “there is so much fear surrounding it.” Anthrax is caused by bacteria that normally live in the soil, but the bugs are also the subject of defense research. People may contract anthrax from inhaling spores, touching the bacteria if they have an open cut, being exposed to dead animals that were infected, or eating infected meat. 

2. HIV

HIV, the human immunodeficiency virus, is transmitted through bodily fluids. There are a couple of factors that impact how infectious the condition is, says Jonathan L. Jacobs, MD, infectious disease specialist at New York-Presbyterian/Weill Cornell Medical Center. One is called viral load, or the amount of virus present in the person with HIV. Shortly after infection and later in the illness, an individual has more of the virus in their blood, and is more infectious. The second variable is the type of exposure. Contact with someone’s saliva brings an almost negligible rate of infection, while a needle stick has a higher infection rate, and receptive anal sex brings the highest rate of sexual exposure. According to the CDC, the highest rate of infection overall is through blood transfusion.

3. Hepatitis C

Hepatitis C is a liver disease caused by a virus that produces symptoms that range from mild liver inflammation to cancer, cirrhosis, and liver failure. The risk of liver cancer and cirrhosis is increased in people with chronic hepatitis C. The virus is transmitted through contact with infected blood, by sharing needles, or from an accidental needle stick. It can also be passed through sexual contact, most commonly between homosexual men, Jacobs said. The disease can be prevented by not sharing needles with someone who is infected and using safe sex practices with partners who may have the condition. The good news, however, is that in the population that requires treatment, antiviral medication cures the condition for over 90 percent of patients. 

4. Hepatitis B

A serious liver infection, hepatitis B is also caused by a virus. In a mild form, it causes inflammation of the liver. But if not treated, it can cause liver cancer or cirrhosis to develop over time. The virus is spread through blood and bodily fluids. It can be transmitted through needle sticks, needle sharing, through sexual intercourse, or from an infected mother to a child during birth. The hepatitis B vaccination is 95 percent effective at preventing infection by the virus, and most health workers are vaccinated.

5. Whooping Cough

Whooping cough, or pertussis, is a highly infectious respiratory condition caused by bacteria. It is spread when someone infected coughs or sneezes near someone who inhales the bacteria. For adults, whooping cough can lead to prolonged periods of uncontrollable coughing. In infants, whooping cough can be serious enough that children can stop breathing.

6. Meningitis

Unlike whooping cough, meningitis is not as highly contagious, but the results of an infection can quickly become catastrophic. Meningitis is caused by a virus or by bacteria that create inflammation in the membranes around the brain and spinal cord, notes the CDC. Transmission of meningitis occurs through exposure to respiratory droplets from an infected person. This can occur when a provider hooks a patient to a ventilator in an emergency room. It is also sometimes seen in areas where people are in close quarters like dorm rooms or soldiers’ barracks.

If left untreated, Tosh said the condition is “almost universally fatal,” so patients need to seek treatment immediately. Antibiotics can be given immediately after exposure to reduce the chance of infection for bacterial meningitis. Fortunately, there is a routine childhood vaccine that can prevent infection. The vaccination is also recommended again for students before going to college.