Normally, the immune system produces white blood cells and antibodies that fight viruses and bacteria. The infection-fighting cells are called T-cell lymphocytes.When only a few organisms are present on or in a tissue, an infection will not necessarily develop. However, when a critical number is exceeded, it is very likely that the tissue will become infected. For every type of microorganism, the minimal infective dose can be determined; this is the lowest number of bacteria, viruses, or fungi that cause the ̃rst clinical signs of infection in a healthy individual. For most causative agents of nosocomial infections, the minimal infective dose is relatively high. For Klebsiella and Serratia spp. and other Enterobacteriaceae, for example,it is more than 100 000, but for hepatitis B virus it is less than 10.
Classification of pathogenic germs:
*Conventional pathogens cause disease in healthy individuals in the absence of specific immunity.Examples:Staphylococcus aureus, Streptococcus pyogenes, Salmonella spp., Shigella spp., Corynebacterium diphtheriae, Mycobacterium tuberculosis, Bordetella pertussis, hepatitis A and B viruses, rubella virus, rotaviruses, human immunodeficiency virus (HIV).
*Conditional pathogens cause disease, other than trivial local infections, only in persons with reduced resistance to infection (including newborn infants) or when implanted directly into tissue or a normally sterile body area. Examples: Streptococcus agalactiae,Enterococcus spp., Clostridium tetani, Escherichia coli, Klebsiella spp., Serratia marcescens, Acinetobacter baumanii, Pseudomonas aeruginosa, Candida spp.
*Opportunistic pathogens cause generalized disease, but only in patients with profoundly diminished resistance to infection. Examples:
atypical mycobacteria, Nocardia asteroides, Pneumocystis carinii.
Virulence refers to the degree of pathogenicity of a microbe, or in other words the relative ability of a microbe to cause disease.The ability of bacteria to cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria called virulence factors. Host-mediated pathogenesis is often important because the host can respond aggressively to infection with the result that host defense mechanisms do damage to host tissues while the infection is being countered.
Viral virulence factors determine whether infection occurs and how severe the resulting viral disease symptoms are. Viruses often require receptor proteins on host cells to which they specifically bind. Typically, these host cell proteins are endocytosed and the bound virus then enters the host cell. Extremely virulent strains can eventually evolve by mutation and natural selection within the virus population inside a host.
Viruses are capsules of genetic material (DNA or RNA). They're much smaller than bacteria. Unlike bacteria, viruses are not "living" organisms. So they require living hosts — such as people, plants or animals — to multiply. When a virus enters your body, it invades some of your cells and takes over the cell machinery, redirecting host cells from their normal function to produce the virus. Viruses may eventually kill their host cells or become part of these cells' genetic material. Some viruses are spread directly from person to person (contagious), such as influenza and the common cold. Other viruses, such as West Nile virus and yellow fever, are not.
Bacteria are single-celled microorganisms that reproduce by dividing. Most bacteria can grow on nonliving surfaces. Not all bacteria are harmful, and some are even beneficial. But when infectious bacteria enter your body, they can make you sick. Bacteria make toxins that can damage specific cells they've invaded. Some bacterial infections are contagious, such as strep throat and tuberculosis. Others — such as bacterial infection of the heart valves (endocarditis) or bone (osteomyelitis) — are not.
Public health is essentially about preparedness, with authorities trying to think one step ahead of the deadly virus or bacteria, as the case may
be.Suppose;Upper respiratory infection (URI) is an imprecise term that covers any infectious-disease process that usually involves the respiratory system starting with the nose and ending just before the lungs.
The infections could be caused by viruses or bacteria, and include conditions such as the common cold, influenza, sinus problems, minor sore throat, and so forth.Taking antibiotics when you do not need them can cause problems when you do need them. Bacteria can change and become able to defend themselves against antibiotics. Those bacteria are called antibiotic resistant.Antibiotics don't help people with the flu get better because they don't work on viruses.
American researchers have found a way to use disease "fingerprints" to identify viruses and bacteria that cause common infections in children.
In some cases, tracking viruses or bacteria that cause illness can be difficult because they may not be present in the blood or other easily accessible areas of the body. To get around this problem, researchers at the University of Texas Southwestern Medical Center, Children's Medical Center Dallas and Baylor Institute for Immunology Research devised a new approach.
It involves analyzing telltale "fingerprints" that a disease leaves behind on cells involved in the body's immune response. Those clues can be used to create a composite sketch of the offending virus or bacteria.
The researchers tested this approach in 29 children with four common infections -- flu, staph, strep, and E. coli -- and were able to distinguish between the flu, strep and E. coli in 95 percent of cases. They were also able to distinguish between staph and E. coli with 85 percent accuracy.
"We are genetically programmed to respond differently to different infections. We have developed the tools to understand that," study lead author Dr. Octavio Ramilo, professor of pediatrics at the University of Texas Southwestern Medical Center, said in a prepared statement.
"Infectious diseases are the No. 1 cause of death in the world. So we hope this eventually can be used not only to diagnose, but also to understand the prognosis and how the body is responding to therapy," Ramilo said.
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