“Pure persons” refers to sinless persons.

When one begins to kill snakes in a lake of snakes, fish are also [incidentally] killed [just] because they live along with the snakes [in that lake]. Similarly [a sinless person is punished for the sins of a sinful person he associates with].¹

NOTE. The principle of avoiding the association of sinful people has been explained by Prabhupāda thus:

In order to be successful in devotional service one must give up the association of undesirable people. This includes karmīs, jñānīs, yogīs and other nondevotees. Once Śrī Caitanya Mahāprabhu was asked by one of His householder devotees about the general principles of Vaiṣṇavism, as well as the general routine activities of a Vaiṣṇava, and Śrī Caitanya Mahāprabhu immediately replied, asat-saṅga-tyāga—ei vaiṣṇava-ācāra: “Characteristically, a Vaiṣṇava is one who gives up the association of worldly people, or nondevotees.” (Cc. Madhya 22.87) Śrīla Narottama Dāsa Ṭhākura has therefore recommended, tāndera caraṇa sevi bhakta-sane vāsa: one has to live in the company of pure devotees and execute the regulative principles laid down by the previous ācāryas, the Six Gosvāmīs (namely, Śrī Rūpa Gosvāmī, Śrī Sanātana Gosvāmī, Śrī Jīva Gosvāmī, Śrī Raghunātha Dāsa Gosvāmī, Śrī Gopāla Bhaṭṭa Gosvāmī and Śrī Raghunātha Bhaṭṭa Gosvāmī). If one lives in the association of devotees, there is little chance of associating with nondevotees. The International Society for Krishna Consciousness is opening many centers just to invite people to live in the company of devotees and practice the regulative principles of spiritual life. (Nectar of Instruction, Verse 3 purport)

The “worldly people,” technically called asat, that Prabhupāda is referring to above are of two types as explained by Lord Caitanya (Caitanya-caritāmṛta, Madhya 22.87):

asat-saṅga-tyāga—ei vaiṣṇava-ācāra
‘strī-saṅgī’—eka asādhu, ‘kṛṣṇābhakta’ āra

“For a Vaiṣṇava, the standard behavior is to avoid the association of ordinary people not interested in kṛṣṇa-bhakti. Such common people are, first, those who are too materially attached, especially to women, and, second, people who are not at all devotees of Lord Kṛṣṇa. Vaiṣṇavas should avoid the company of these two kinds of people.”

Lord Caitanya then presents the following verse from Śrīmad-Bhāgavatam as evidence to avoid the first type of asat:

satyaṁ śaucaṁ dayā maunaṁ buddhir hrīḥ śrīr yaśaḥ kṣamā
śamo damo bhagaś ceti yat-saṅgād yāti saṅkṣayam

teṣv aśānteṣu mūḍheṣu khaṇḍitātmasv asādhuṣu
saṅgaṁ na kuryāc chocyeṣu yoṣit-krīḍā-mṛgeṣu ca

na tathāsya bhaven moho bandhaś cānya-prasaṅgataḥ
yoṣit-saṅgād yathā puṁso yathā tat-saṅgi-saṅgataḥ

“By association with worldly people, one becomes devoid of truthfulness, cleanliness, mercy, gravity, spiritual intelligence, shyness, austerity, fame, forgiveness, control of the mind, control of the senses, fortune and all opportunities. One should not at any time associate with a coarse fool who is bereft of the knowledge of self-realization and who is no more than a toy animal in the hands of a woman. The illusion and bondage that accrue to a man from attachment to any other object are not as complete as that resulting from association with a woman or with men too much attached to women.”²

Prabhupāda’s purport to this verse is as follows.

These verses, quoted from Śrīmad-Bhāgavatam (3.31.33-35), were spoken by Kapiladeva, an incarnation of the Supreme Personality of Godhead, to His mother. Herein Kapiladeva discusses pious and impious activities and the symptoms of those who are devoid of devotional service to Kṛṣṇa. Generally people do not know about the miserable conditions within the womb of a mother in any species of life. Due to bad association, one gradually falls into lower species. Association with women is greatly stressed in this regard. When one becomes attached to women or to those who are attached to women, one falls down into the lower species.

puruṣaḥ prakṛti-stho hi bhuṅkte prakṛti-jān guṇān
kāraṇaṁ guṇa-saṅgo ’sya sad-asad-yoni-janmasu

“The living entity in material nature thus follows the ways of life, enjoying the three modes of nature. This is due to his association with that material nature. Thus he meets with good and evil among various species.” (Bhagavad-gītā 13.22)

According to Vedic civilization, one’s association with women should be very much restricted. In spiritual life there are four āśramas—brahmacarya, gṛhastha, vānaprastha and sannyāsa. The brahmacārī, vānaprastha and sannyāsī are completely forbidden to associate with women. Only gṛhasthas are allowed to associate with women under certain very much restricted conditions—that is, one  associates with women to propagate nice children. Other reasons for association are condemned.³

Śrī Caitanya Mahāprabhu then quotes the following verse from Kātyāyana-saṁhitā as evidence to avoid the second type of asat:

varaṁ huta-vaha-jvālā-pañjarāntar-vyavasthitiḥ
na śauri-cintā-vimukha-jana-saṁvāsa-vaiśasam

“It is better to accept the miseries of being encaged within bars and surrounded by burning flames than to associate with those bereft of Kṛṣṇa consciousness. Such association is a very great hardship.”⁴

On the other hand, associating with pure devotees of the Supreme Personality of Godhead will help one clearly understand one’s relationship with the Supreme Lord and his duties towards Him:

To understand Kṛṣṇa consciousness and action according to its modes, one has to learn one’s relationship with the Supreme; i.e., one who has learned perfectly knows that every living entity is an eternal servitor of the Lord and that consequently one has to act in Kṛṣṇa consciousness. The entire Bhagavad-gītā is directed toward this conclusion. Any other conclusions, against this consciousness and its attendant actions, are vikarmas, or prohibited actions. To understand all this one has to associate with authorities in Kṛṣṇa consciousness and learn the secret from them; this is as good as learning from the Lord directly. (Bhagavad-gītā 4.17 purport)