The Relationship Between Pathogens and Humans
Infectious diseases are transmissible, or communicable, diseases. This means that these diseases are caused by pathogens that can spread from infected people to uninfected people. Some diseases can only spread from one person to another by direct contact as the pathogen cannot survive outside the human body. Others can survive in water, human food, feces or animals (including insects) and so are transmitted indirectly from person to person.
- Cholera is caused by the bacterium Vibrio cholerae and is transmitted in water or food contaminated by the feces of infected people
- Cholera can be controlled by treating patients with oral re-hydration therapy and making sure that human feces do not reach the water supply. this disease is prevented by providing clean, chlorinated water and good sanitation. there is no effective vaccine.
- Malaria is caused by four species of Plasmodium. The most dangerous is P. falciparum. The disease is transmitted by female Anopheles mosquitoes that transfer Plasmodium from infected to uninfected people.
- Malaria is controlled in three ways: reducing the number of mosquitoes by insecticide spraying or draining breeding sites; using mosquito nets (more effective if soaked in insecticide); using more drugs to prevent Plasmodium from infecting people. There is no effective vaccine.
- AIDS is a set of diseases caused by the destruction of the immune system by infection with human immunodeficiency virus (HIV). HIV is transmitted in certain body fluids: blood, semen, vaginal secretions, and breast milk. It also crosses the placenta. It primarily infects economically active members in populations in developing countries and has an extremely adverse effect on social and economic development.
- The transmission of HIV can be controlled by using barrier methods (e.g. condom) during sexual intercourse. Educating people to practice safer sex is the only control method currently available to health authorities. Contact tracing is used to find people who may have contracted HIV so that they can be tested and counselled. Life expectancy can be greatly extended by the use of combination drug therapy of the virus. However, such treatment is expensive, difficult to adhere to and has unpleasant side effects. There is no vaccine for HIV and no cure for AIDS.
- TB is caused by the bacterium Mycobaterium tuberculosis (in developing countries, of may also be caused by M. bovis, which also causes a related disease in cattle).
- M. tuberculosis is spread when people infected with the active form of disease release bacteria in droplets of liquid when they cough or sneeze. Transmission occurs when uninfected people live in over crowed conditions and especially where many sleep close together. Many people have the inactive form of TB in their lungs, but they do not have the disease and do not spread it. The inactive bacteria may become active in people who are malnourished or who become infected with HIV.
- Drugs are used to treat people with the active form of TB. The treatment may take nine months or more as it is difficult to kill the bacteria. Contract tracing is used to find people who may have the disease. These people are tested for TB and treated of found to be infected. The BCG vaccine provides some protection against TB, but its effectiveness varies in different parts of the world.
- Cholera, malaria, AIDS, and TB are all increasing in prevalence and pose severe threats to the health of populations in developed and developing countries.
- Public health measures are taken to reduce the transmission of these disease, but to be effective they must be informed by a knowledge of the life cycle of each pathogen.
- Antibiotics are used to inhibit the growth of pathogenic organisms, Most are only effective against bacteria. The widespread and indiscriminate use of antibiotics has led to the growth of resistant strains of bacteria. This poses a serious challenge to the maintenance of health services in the 21st century.
Immunity
- Phagocytes and lymphocytes are the cells of the immune system.
- Phagocytes originate in the bone marrow and are produced there throughout life. There are two types: neutrophils circulate in the blood and enter infected tissues; macrophages are more stationary inside tissues. They destroy bacteria and viruses by phagocytosis.
- Lymphocytes also originate in bone marrow, but migrate just before and after birth to other sites in the body. There are two types: B lymphocytes (B cells) and T lymphocytes (T cells).
- Antigens are 'foreign' macromolecules that stimulate the immune system.
- During an immune response, those B and T cells that have receptors specific to the antigen are activated.
- When B cells are activated they form plasma cells which secrete antibodies.
- T lymphocytes do not secrete antibodies; their surface receptors are similar to antibodies and identify antigens. They mature in the thymus and develop into T helper cells or killer T cells (cytotoxic T cells). T helper cells secrete cytokines that control the immune system, activating B cells and killer T cells, which kill infected host cells.
- During an immune response, memory cells are formed which retain the ability to divide rapidly and develop into active B or T cells on a second exposure to the same antigen (immunological memory).
- Antibodies are globular glycoproteins. They all have one or more pairs of identical heavy polypeptides and of identical light polypeptides. Each type of antibody interacts with one antigen via the specific shape of its variable region. Each molecule of the simplest antibody can bind to two antigen molecules. Larger antibodies have more than two antigen binding sites.
- Antibodies agglutinate bacteria; prevent viruses infecting cells; coat bacteria and viruses to aid phagocytosis; act with plasma proteins to burst bacteria; neutralize toxins.
- Active immunity is the production of antibodies and active T cells during a primary immune response to an antigen acquired either naturally by infection or artificially by vaccination. This gives permanent immunity.
- Passive immunity is the introduction of antibodies either naturally across the placenta or in breast milk, or artificially by injection.
- Vaccination confers artificial active immunity by introducing a small quantity of an antigen by injection or by mouth. This may be a whole living organism, a dead one, a harmless version of a toxin, or a preparation of surface antigens.
- It is difficult to develop successful vaccines against diseases caused by organisms that have many different strains, or express different antigens during their life cycle within humans, or infect parts or the party beyond the reach of antibodies (antigenic concealment).
- Smallpox was eradicated by a programme of surveillance, contact tracing and 'ring' vaccination, using a 'live' vaccine against the only strain of the smallpox virus.
- Measles is a common cause of death amongst infants in poor communities. It is difficult to eradicate because a wide coverage of vaccination has not been achieved and malnourished children do not respond well to just one dose of the vaccine.