Bacteria, viruses, parasites and fungi live in and on us all the time. Most are harmless, but certain species are pathogens - they can cause an illness if a change in conditions allows them to thrive. Other diseases are passed to us from people or animals. A fever is almost always a sign that an infection is taking hold.
Unwanted visitors
Organisms that live off the body's cells or tissues are called parasites. There are five main types: bacteria, viruses, fungi and animals and protozoans. When they find favourable conditions they multiply rapidly but may produce harmful products or effects that make us feel ill, prompting our immune system to swing into action.
How diseases spread
There are many infectious diseases but some affect relatively few individuals and are local to a small area - only diseases that spread easily by person-to-person contact are said to be contagious. Many pathogens travel between people by less direct means - through the air or in water, on objects someone has touched, or in contaminated food. Zoonotic diseases are animal infections that can spread to humans, usually through bites.
Bacteria
Bacteria are microscopic organisms that are usually harmless, but can sometimes cause disease. Bacteria are responsible for some globally important diseases such as tuberculosis and pneumonia.
SALMONELLA (food poisoning)
VIBRIO (cholera)
TREPONEMA (yaws, syphilis)
STREPTOCOCCUS (pneumonia, bronchitis)
Commonly used for bacterial infections, antibiotics break down the walls of bacteria or interrupt their growth. However, they can't distinguish the good bacteria from the bad.
Viruses
Viruses are the smallest and simplest organisms of all, made up of only their genetic material (DNA or RNA) in a protein coat. Unlike other pathogens, viruses need the host's cells to live and replicate.
ADENOVIRUS (tonsilitis, conjunctivitis)
LYSSAVIRUS (rabies)
LENTIVIRUS (HIV/AIDS)
Vaccination
The best way of preventing the spread of viral infections is through vaccination. A vaccine primes the immune system to recognise the virus and launch an immediate attack.
Fungi
Fungi are always present in and on the body, but sometimes pathogenic species take hold and cause diseases such as athletes foot or thrush.
COCCIDIOIDES (valley fever)
CRYPTOCOCCUS (lung or meningeal cryptococcus)
Antifungal medications
Fungal infections are treated according to whether they are internal or external. The active ingredients either attack the fungus directly by breaking down its cell walls, or prevent it from growing.
Animals and protozoans
We also face attacks from tiny animals and single-celled organisms called protozoans that live on or inside the body. Some are large enough to see with the naked eye, such as worms, or they may be microscopic, such as Giardia, the protozoan that causes diarrhoea.
NEMATODE (Guniea worm, thread worm)
GIARDIA (diarrhoea)
Prevention
The best strategy against this type of infection is to avoid activities and areas where there are known health hazards, be wary of unsafe food and water sources, and to take recommended precautionary drugs.
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