In the active voice, the subject of the sentence is the actor:

Malaria threatens us.

In the passive voice, the subject of the sentence is acted upon:

We are threatened by malaria.

Ordinarily, your writing will be more direct and forceful when you use the active voice:

More than 200 million people a year are sickened by malaria.

To make a passive sentence active, you don’t always need to turn it around the way we did in the pair of examples above. Sometimes you can simply change the verb:

The passive voice is fine in situations in which you do not want to identify the actor or in which the actor is not important. Here, for example, identifying who uses the insecticides is not necessary:

Insecticides are used in bed nets and sprays to kill mosquitoes.

The passive voice can also be useful when it allows you to start or end a sentence in a way that flows smoothly from the previous sentence into the next one. For instance, compare these two passages:

Malaria poses a serious problem for public health. A parasite causes malaria, and mosquitoes spread it. Killing the parasite, the mosquitoes, or both combats the disease.

Malaria poses a serious problem for public health. It is caused by a parasite, which is spread by mosquitoes. It can be combated by killing the parasite, the mosquitoes, or both.

Although all the verbs in the first passage are in the active voice, the train of thought seems disorganized, jumping from malaria to the parasite and back to malaria, to the mosquitoes and back to malaria, and to killing the parasite and the mosquitoes and back to malaria. In the second passage, malaria is the subject of all three sentences, making it clear that the passage is consistently about the disease.