What Causes Disease?

Bacteria

Bacteria are microscopic single-celled organisms that thrive in diverse environments. Bacteria are prokaryotes. The entire organism consists of a single cell with a simple internal structure. They can live within soil, in the ocean and inside the human gut. Humans' relationship with bacteria is complex.

Sometimes they lend a helping hand, by curdling milk into yogurt, or helping with our digestion. At other times they are destructive, causing diseases like pneumonia and urine infections.

Viruses

Viruses are very tiny agents. They are made of genetic material inside of a protein coating. Viruses cause familiar infectious diseases such as the common cold, flu and warts. They also cause severe illnesses such as HIV/AIDS and Ebola.

Viruses are like hijackers. They invade living, normal cells and use those cells to multiply and produce other viruses like themselves. This can kill, damage or change the cells and make you sick. Different viruses attack certain cells in your body such as your liver, respiratory system or blood.

When you get a virus, you may not always get sick from it. Your immune system may be able to fight it off.

For most viral infections, treatments can only help with symptoms while you wait for your immune system to fight off the virus. Antibiotics do not work for viral infections. There are antiviral medicines to treat some viral infections. Vaccines can help prevent you from getting many viral diseases.

Parasites

A parasite is an organism that lives on or in a host. It gets its food from, or at the expense of, its host.

Parasites can cause or transmit disease in humans termed as a ‘parasitic disease’ or ‘parasitosis’. Many parasites do not cause disease.

Infestation refers to parasitic diseases caused by lice, fleas, ticks, and some mites, that live on a host. It can also refer to worms and protozoa that live in the internal organs or tissues of its host.

‘Parasitic disease’ caused by worms and protozoa are sometimes referred to as ‘parasitic infection’.

Fungal infections

A fungus is a primitive organism. Mushrooms, mould and mildew are examples. Fungi live in air, in soil, on plants and in water. Some live in the human body. Only about half of all types of fungi are harmful.

Some fungi reproduce through tiny spores in the air. You can inhale the spores, or they can land on you. As a result, fungal infections often start in the lungs or on the skin. You are more likely to get a fungal infection if you have a weakened immune system or take antibiotics.

Fungi can be difficult to kill. For skin and nail infections, you can apply medicine directly to the infected area. Oral antifungal medicines are also available for serious infections.